diff --git a/Cargo.lock b/Cargo.lock
index fef0d0c..3c654da 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -470,7 +470,7 @@ dependencies = [
 
 [[package]]
 name = "guestbook"
-version = "0.2.10"
+version = "0.2.11"
 dependencies = [
  "axum",
  "base64 0.22.1",
diff --git a/Cargo.toml b/Cargo.toml
index c7ab141..5d3f9fc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "guestbook"
-version = "0.2.10"
+version = "0.2.11"
 edition = "2021"
 description = "A configurable, self-hosted guestbook for the web, allowing visitors to leave behind messages, drawings, and voice notes, with spam-prevention and moderation via Telegram bot."
 license = "MIT"
diff --git a/README.md b/README.md
index c794716..3ffc5b9 100644
--- a/README.md
+++ b/README.md
@@ -191,15 +191,21 @@ Running `guestbook` with no env vars will give you a working guestbook on `local
 # Message textarea height in pixels.
 # BOOK_TEXTAREA_HEIGHT=150
 
-# Custom HTML template file with {{title}}, {{form}}, {{entries}}, and {{style}} placeholders.
+# Custom HTML template file with {{title}}, {{form}}, {{entries}}, {{style}}, and {{base}} placeholders.
 # Uses built-in default if unset.
 # BOOK_TEMPLATE=./templates/default.html
 
 # Custom success page template shown after a successful submission.
-# Supports {{title}} and {{style}} placeholders. Use <script> for dynamic behavior.
+# Supports {{title}}, {{style}}, and {{base}} placeholders. Use <script> for dynamic behavior.
 # Uses built-in templates/success.html if unset.
 # BOOK_SUCCESS_TEMPLATE=./templates/success.html
 
+# URL prefix the guestbook is mounted at. Empty serves at the domain root.
+# When set (e.g. /guestbook), all routes (/, /submit, /drawings/*, /voice_notes/*)
+# are mounted under the prefix, and form actions and asset URLs include it.
+# Templates can interpolate the prefix with the {{base}} placeholder.
+# BOOK_BASE_PATH=
+
 # Enable drawing canvas in submission form. Drawings are stored as PNG files in DATA_DIR/drawings/.
 # BOOK_ENABLE_DRAWINGS=false
 
@@ -430,6 +436,8 @@ entered into the 'message' field.
     entries - Approved guestbook entries, newest first.
     style   - Custom CSS from BOOK_STYLE or BOOK_STYLE_FILE, wrapped in
               a <style> tag. Uses built-in default.css when neither is set.
+    base    - URL prefix the guestbook is mounted at (BOOK_BASE_PATH).
+              Empty when serving at the domain root.
 
   See default.css for available CSS classes on rendered elements.
 -->
@@ -468,6 +476,7 @@ entered into the 'message' field.
 
     title - Site title (BOOK_SITE_TITLE).
     style - Custom CSS (same as the main template).
+    base  - URL prefix the guestbook is mounted at (BOOK_BASE_PATH).
 
   Everything else is static — write whatever you want. Use <script> for
   dynamic behavior like showing the current time.
@@ -483,7 +492,7 @@ entered into the 'message' field.
 <body>
 <div class="page-container">
 <p>Thanks! Your message is pending approval.</p>
-<p><a href="/">&#8592; back</a></p>
+<p><a href="{{base}}/">&#8592; back</a></p>
 </div>
 </body>
 </html>
diff --git a/module.nix b/module.nix
index f8f0b27..6b73ca4 100644
--- a/module.nix
+++ b/module.nix
@@ -30,6 +30,18 @@ in
       description = "Site title shown in nav and page title.";
     };
 
+    basePath = mkOption {
+      type = types.str;
+      default = "";
+      example = "/guestbook";
+      description = ''
+        URL prefix the guestbook is mounted at. Empty serves at the domain root.
+        When set, all routes (/, /submit, /drawings/*, /voice_notes/*) are
+        mounted under the prefix, and form actions and asset URLs include it.
+        Templates can interpolate the prefix with the {{base}} placeholder.
+      '';
+    };
+
     user = mkOption {
       type = types.str;
       default = "guestbook";
@@ -268,13 +280,13 @@ in
       templateFile = mkOption {
         type = types.nullOr types.path;
         default = null;
-        description = "Custom HTML template file with {{title}}, {{form}}, {{entries}}, and {{style}} placeholders. Uses built-in default if null.";
+        description = "Custom HTML template file with {{title}}, {{form}}, {{entries}}, {{style}}, and {{base}} placeholders. Uses built-in default if null.";
       };
 
       successTemplateFile = mkOption {
         type = types.nullOr types.path;
         default = null;
-        description = "Custom success page template with {{title}} and {{style}} placeholders. Uses built-in default if null.";
+        description = "Custom success page template with {{title}}, {{style}}, and {{base}} placeholders. Uses built-in default if null.";
       };
 
       labels = {
@@ -380,6 +392,7 @@ in
           BOOK_CONTENT_REQUIRED = if cfg.content.required then "true" else "false";
           BOOK_TEXTAREA_WIDTH = toString cfg.styles.message.width;
           BOOK_TEXTAREA_HEIGHT = toString cfg.styles.message.height;
+          BOOK_BASE_PATH = cfg.basePath;
         } // lib.optionalAttrs (cfg.styles.cssFile != null) {
           BOOK_STYLE_FILE = cfg.styles.cssFile;
         } // lib.optionalAttrs (cfg.styles.templateFile != null) {
diff --git a/src/config.rs b/src/config.rs
index 67e759c..7607cda 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -50,6 +50,7 @@ pub struct Config {
     pub voice_note_record_text: String,
     pub textarea_width: u32,
     pub textarea_height: u32,
+    pub base_path: String,
 }
 
 impl Config {
@@ -206,10 +207,22 @@ impl Config {
                 .unwrap_or_else(|_| "150".into())
                 .parse()
                 .map_err(|_| "BOOK_TEXTAREA_HEIGHT must be a number")?,
+            base_path: normalise_base_path(env::var("BOOK_BASE_PATH").unwrap_or_default()),
         })
     }
 }
 
+fn normalise_base_path(raw: String) -> String {
+    let trimmed = raw.trim().trim_end_matches('/');
+    if trimmed.is_empty() {
+        String::new()
+    } else if trimmed.starts_with('/') {
+        trimmed.to_string()
+    } else {
+        format!("/{trimmed}")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -367,4 +380,54 @@ mod tests {
         let config = Config::from_env().unwrap();
         assert!(config.success_template.is_none());
     }
+
+    #[test]
+    fn test_base_path_default_empty() {
+        let _lock = ENV_LOCK.lock().unwrap();
+        env::remove_var("BOOK_BASE_PATH");
+        env::remove_var("BOOK_TELEGRAM_BOT_TOKEN");
+        env::remove_var("BOOK_TELEGRAM_CHAT_ID");
+
+        let config = Config::from_env().unwrap();
+        assert_eq!(config.base_path, "");
+    }
+
+    #[test]
+    fn test_base_path_from_env() {
+        let _lock = ENV_LOCK.lock().unwrap();
+        env::set_var("BOOK_BASE_PATH", "/guestbook");
+        env::remove_var("BOOK_TELEGRAM_BOT_TOKEN");
+        env::remove_var("BOOK_TELEGRAM_CHAT_ID");
+
+        let config = Config::from_env().unwrap();
+        assert_eq!(config.base_path, "/guestbook");
+
+        env::remove_var("BOOK_BASE_PATH");
+    }
+
+    #[test]
+    fn test_base_path_normalises_trailing_slash() {
+        let _lock = ENV_LOCK.lock().unwrap();
+        env::set_var("BOOK_BASE_PATH", "/guestbook/");
+        env::remove_var("BOOK_TELEGRAM_BOT_TOKEN");
+        env::remove_var("BOOK_TELEGRAM_CHAT_ID");
+
+        let config = Config::from_env().unwrap();
+        assert_eq!(config.base_path, "/guestbook");
+
+        env::remove_var("BOOK_BASE_PATH");
+    }
+
+    #[test]
+    fn test_base_path_normalises_missing_leading_slash() {
+        let _lock = ENV_LOCK.lock().unwrap();
+        env::set_var("BOOK_BASE_PATH", "guestbook");
+        env::remove_var("BOOK_TELEGRAM_BOT_TOKEN");
+        env::remove_var("BOOK_TELEGRAM_CHAT_ID");
+
+        let config = Config::from_env().unwrap();
+        assert_eq!(config.base_path, "/guestbook");
+
+        env::remove_var("BOOK_BASE_PATH");
+    }
 }
diff --git a/src/render.rs b/src/render.rs
index 312a888..e19d479 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -18,6 +18,7 @@ pub fn render_page(template: &str, config: &Config, entries: &[Entry], form_html
         .replace("{{form}}", form_html)
         .replace("{{entries}}", &entries_html)
         .replace("{{style}}", &style)
+        .replace("{{base}}", &config.base_path)
 }
 
 pub fn render_form(config: &Config) -> String {
@@ -170,13 +171,14 @@ pub fn render_form(config: &Config) -> String {
     };
 
     format!(
-        r#"<form class="guestbook-form" method="post" action="/submit" accept-charset="UTF-8">
+        r#"<form class="guestbook-form" method="post" action="{base}/submit" accept-charset="UTF-8">
 <label class="guestbook-label" for="name">{label_name}</label>
 <input class="guestbook-input" id="name" name="name" required>
 {website_section}<label class="guestbook-label" for="message">{label_message}</label>
 <textarea class="guestbook-textarea" id="message" name="message" style="width:{tw}px;height:{th}px"></textarea>
 {drawing_section}{voice_note_section}{captcha_section}<input name="url" aria-hidden="true" style="position:absolute;width:1px;height:1px;overflow:hidden;clip:rect(0,0,0,0)" tabindex="-1" autocomplete="off"><button class="guestbook-button" type="submit">{button}</button>
 </form>"#,
+        base = config.base_path,
         label_name = config.label_name,
         website_section = website_section,
         label_message = config.label_message,
@@ -200,6 +202,7 @@ pub fn render_success_page(config: &Config) -> String {
     template
         .replace("{{title}}", &config.site_title)
         .replace("{{style}}", &style)
+        .replace("{{base}}", &config.base_path)
 }
 
 pub fn render_error_page(config: &Config, error: &str) -> String {
@@ -223,11 +226,12 @@ pub fn render_error_page(config: &Config, error: &str) -> String {
 <body>
 <div class="page-container">
 <p>{error}</p>
-<p><a href="/">&#8592; back</a></p>
+<p><a href="{base}/">&#8592; back</a></p>
 </div>
 </body>
 </html>"#,
         title = config.site_title,
+        base = config.base_path,
     )
 }
 
@@ -273,7 +277,8 @@ fn render_entry(entry: &Entry, config: &Config) -> String {
     };
     let drawing_html = if !entry.meta.drawing.is_empty() {
         format!(
-            "<dd class=\"entry-drawing-wrap\"><img class=\"entry-drawing\" src=\"/drawings/{}\" alt=\"Drawing by {}\"></dd>",
+            "<dd class=\"entry-drawing-wrap\"><img class=\"entry-drawing\" src=\"{}/drawings/{}\" alt=\"Drawing by {}\"></dd>",
+            config.base_path,
             escape_html(&entry.meta.drawing),
             escape_html(&entry.meta.name)
         )
@@ -282,7 +287,8 @@ fn render_entry(entry: &Entry, config: &Config) -> String {
     };
     let voice_note_html = if !entry.meta.voice_note.is_empty() {
         format!(
-            "<dd class=\"entry-voice-note-wrap\"><audio controls preload=\"metadata\" src=\"/voice_notes/{}\"></audio></dd>",
+            "<dd class=\"entry-voice-note-wrap\"><audio controls preload=\"metadata\" src=\"{}/voice_notes/{}\"></audio></dd>",
+            config.base_path,
             escape_html(&entry.meta.voice_note)
         )
     } else {
@@ -352,6 +358,7 @@ mod tests {
             voice_note_record_text: "record".into(),
             textarea_width: 400,
             textarea_height: 150,
+            base_path: String::new(),
         }
     }
 
@@ -511,6 +518,73 @@ mod tests {
         assert!(!html.contains("<script>alert("));
     }
 
+    #[test]
+    fn test_render_form_uses_base_path_for_submit() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        let form = render_form(&config);
+        assert!(form.contains("action=\"/guestbook/submit\""));
+        assert!(!form.contains("action=\"/submit\""));
+    }
+
+    #[test]
+    fn test_render_form_default_submit_path_unchanged() {
+        let config = test_config();
+        let form = render_form(&config);
+        assert!(form.contains("action=\"/submit\""));
+    }
+
+    #[test]
+    fn test_render_entry_drawing_uses_base_path() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        let mut entry = make_entry("alice", "2026-04-09", "");
+        entry.meta.drawing = "abc.png".into();
+        let html = render_page(DEFAULT_TEMPLATE, &config, &[entry], "");
+        assert!(html.contains("src=\"/guestbook/drawings/abc.png\""));
+        assert!(!html.contains("src=\"/drawings/abc.png\""));
+    }
+
+    #[test]
+    fn test_render_entry_voice_note_uses_base_path() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        let mut entry = make_entry("bob", "2026-04-09", "");
+        entry.meta.voice_note = "1.webm".into();
+        let html = render_page(DEFAULT_TEMPLATE, &config, &[entry], "");
+        assert!(html.contains("src=\"/guestbook/voice_notes/1.webm\""));
+        assert!(!html.contains("src=\"/voice_notes/1.webm\""));
+    }
+
+    #[test]
+    fn test_render_page_substitutes_base_placeholder() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        let custom = "<html><a href=\"{{base}}/\">home</a> {{form}} {{entries}} {{style}} {{title}}</html>";
+        let html = render_page(custom, &config, &[], "");
+        assert!(html.contains("href=\"/guestbook/\""));
+        assert!(!html.contains("{{base}}"));
+    }
+
+    #[test]
+    fn test_render_success_page_substitutes_base_placeholder() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        config.success_template = Some("<a href=\"{{base}}/\">back</a> {{title}} {{style}}".into());
+        let html = render_success_page(&config);
+        assert!(html.contains("href=\"/guestbook/\""));
+        assert!(!html.contains("{{base}}"));
+    }
+
+    #[test]
+    fn test_render_error_page_back_link_uses_base_path() {
+        let mut config = test_config();
+        config.base_path = "/guestbook".into();
+        let html = render_error_page(&config, "oops");
+        assert!(html.contains("href=\"/guestbook/\""));
+        assert!(!html.contains("href=\"/\""));
+    }
+
     #[test]
     fn test_render_entry_preserves_html_when_injection_enabled() {
         let mut config = test_config();
diff --git a/src/web.rs b/src/web.rs
index b8d1790..3809782 100644
--- a/src/web.rs
+++ b/src/web.rs
@@ -39,11 +39,19 @@ pub struct SubmitForm {
 }
 
 pub fn router(state: Arc<AppState>) -> Router {
-    Router::new()
+    let inner: Router<Arc<AppState>> = Router::new()
         .route("/", get(index))
         .route("/submit", post(submit))
         .route("/drawings/{filename}", get(serve_drawing))
-        .route("/voice_notes/{filename}", get(serve_voice_note))
+        .route("/voice_notes/{filename}", get(serve_voice_note));
+    let routed: Router<Arc<AppState>> = if state.config.base_path.is_empty() {
+        inner
+    } else {
+        Router::new()
+            .route(&format!("{}/", state.config.base_path), get(index))
+            .nest(&state.config.base_path, inner)
+    };
+    routed
         .layer(DefaultBodyLimit::max(2 * 1024 * 1024))
         .layer(middleware::from_fn(security_headers))
         .with_state(state)
@@ -440,6 +448,7 @@ mod tests {
             voice_note_record_text: "record".into(),
             textarea_width: 400,
             textarea_height: 150,
+            base_path: String::new(),
         }
     }
 
@@ -472,6 +481,72 @@ mod tests {
         String::from_utf8(bytes.to_vec()).unwrap()
     }
 
+    async fn get_status(app: &Router, uri: &str) -> StatusCode {
+        let req = Request::builder().uri(uri).body(Body::empty()).unwrap();
+        app.clone().oneshot(req).await.unwrap().status()
+    }
+
+    async fn post_form_at(app: &Router, uri: &str, body: &str) -> (StatusCode, String) {
+        let req = Request::builder()
+            .method("POST")
+            .uri(uri)
+            .header("content-type", "application/x-www-form-urlencoded")
+            .body(Body::from(body.to_string()))
+            .unwrap();
+        let resp = app.clone().oneshot(req).await.unwrap();
+        let status = resp.status();
+        let bytes = resp.into_body().collect().await.unwrap().to_bytes();
+        (status, String::from_utf8(bytes.to_vec()).unwrap())
+    }
+
+    #[tokio::test]
+    async fn test_base_path_serves_index_at_prefix_with_slash() {
+        let dir = tempfile::tempdir().unwrap();
+        let mut config = test_config(dir.path());
+        config.base_path = "/guestbook".into();
+        let (app, _rx) = test_app(config);
+        assert_eq!(get_status(&app, "/guestbook/").await, StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn test_base_path_serves_index_at_prefix_no_slash() {
+        let dir = tempfile::tempdir().unwrap();
+        let mut config = test_config(dir.path());
+        config.base_path = "/guestbook".into();
+        let (app, _rx) = test_app(config);
+        assert_eq!(get_status(&app, "/guestbook").await, StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn test_base_path_root_returns_404() {
+        let dir = tempfile::tempdir().unwrap();
+        let mut config = test_config(dir.path());
+        config.base_path = "/guestbook".into();
+        let (app, _rx) = test_app(config);
+        assert_eq!(get_status(&app, "/").await, StatusCode::NOT_FOUND);
+    }
+
+    #[tokio::test]
+    async fn test_base_path_accepts_submit_at_prefix() {
+        let dir = tempfile::tempdir().unwrap();
+        let mut config = test_config(dir.path());
+        config.base_path = "/guestbook".into();
+        let (app, _rx) = test_app(config);
+        let (status, body) = post_form_at(&app, "/guestbook/submit", "name=user&message=hi").await;
+        assert_eq!(status, StatusCode::OK);
+        assert!(body.contains("pending approval"));
+    }
+
+    #[tokio::test]
+    async fn test_base_path_rejects_submit_at_root() {
+        let dir = tempfile::tempdir().unwrap();
+        let mut config = test_config(dir.path());
+        config.base_path = "/guestbook".into();
+        let (app, _rx) = test_app(config);
+        let (status, _) = post_form_at(&app, "/submit", "name=user&message=hi").await;
+        assert_eq!(status, StatusCode::NOT_FOUND);
+    }
+
     #[tokio::test]
     async fn test_enable_submissions_shows_form() {
         let dir = tempfile::tempdir().unwrap();
diff --git a/templates/default.html b/templates/default.html
index 4cca25b..a88b8b3 100644
--- a/templates/default.html
+++ b/templates/default.html
@@ -15,6 +15,8 @@
     entries - Approved guestbook entries, newest first.
     style   - Custom CSS from BOOK_STYLE or BOOK_STYLE_FILE, wrapped in
               a <style> tag. Uses built-in default.css when neither is set.
+    base    - URL prefix the guestbook is mounted at (BOOK_BASE_PATH).
+              Empty when serving at the domain root.
 
   See default.css for available CSS classes on rendered elements.
 -->
diff --git a/templates/success.html b/templates/success.html
index f592a07..44dfb7d 100644
--- a/templates/success.html
+++ b/templates/success.html
@@ -6,6 +6,7 @@
 
     title - Site title (BOOK_SITE_TITLE).
     style - Custom CSS (same as the main template).
+    base  - URL prefix the guestbook is mounted at (BOOK_BASE_PATH).
 
   Everything else is static — write whatever you want. Use <script> for
   dynamic behavior like showing the current time.
@@ -21,7 +22,7 @@
 <body>
 <div class="page-container">
 <p>Thanks! Your message is pending approval.</p>
-<p><a href="/">&#8592; back</a></p>
+<p><a href="{{base}}/">&#8592; back</a></p>
 </div>
 </body>
 </html>
