diff --git a/app/__init__.py b/app/__init__.py
index a20d23c..3961366 100644
--- a/app/__init__.py
+++ b/app/__init__.py
@@ -27,10 +27,12 @@ def create_app():
     from .routes.auth import auth_bp
     from .routes.main import main_bp
     from .routes.friends import friends_bp
+    from .routes.presence import presence_bp
 
     app.register_blueprint(main_bp)
     app.register_blueprint(auth_bp)
     app.register_blueprint(friends_bp)
+    app.register_blueprint(presence_bp)
 
     init_db(app)
 
diff --git a/app/db.py b/app/db.py
index d0f9ae3..b14ff1a 100644
--- a/app/db.py
+++ b/app/db.py
@@ -66,4 +66,13 @@ def init_db(app):
         );
         """
         )
-        db.commit()
\ No newline at end of file
+
+        # Lightweight migration support for existing databases.
+        user_columns = {
+            row["name"]
+            for row in db.execute("PRAGMA table_info(users)").fetchall()
+        }
+        if "last_seen_at" not in user_columns:
+            db.execute("ALTER TABLE users ADD COLUMN last_seen_at TIMESTAMP")
+
+        db.commit()
diff --git a/app/routes/presence.py b/app/routes/presence.py
new file mode 100644
index 0000000..26f7210
--- /dev/null
+++ b/app/routes/presence.py
@@ -0,0 +1,24 @@
+from flask import Blueprint, jsonify
+from flask_login import login_required, current_user
+from app.db import get_db
+
+presence_bp = Blueprint("presence", __name__)
+
+def _mark_current_user_online() -> None:
+    db = get_db()
+    db.execute(
+        "UPDATE users SET last_seen_at = CURRENT_TIMESTAMP WHERE id = ?",
+        (current_user.id,),
+    )
+    db.commit()
+
+
+@presence_bp.route("/api/presence/ping", methods=["POST"])
+@login_required
+def presence_ping():
+    if not current_user.is_authenticated: # should not happen due to @login_required, but just in case
+        return ("", 204)
+
+    _mark_current_user_online()
+    return jsonify({"status": "ok"})
+    
\ No newline at end of file
diff --git a/app/static/js/presence.js b/app/static/js/presence.js
new file mode 100644
index 0000000..e036a69
--- /dev/null
+++ b/app/static/js/presence.js
@@ -0,0 +1,18 @@
+(function startPresenceHeartbeat() {
+  const ping = function () {
+    fetch("/api/presence/ping", {
+      method: "POST",
+      credentials: "same-origin",
+      keepalive: true,
+      headers: {
+        "Content-Type": "application/json",
+      },
+      body: "{}",
+    }).catch(function () {
+      // ignore network issues; next interval will retry
+    });
+  };
+
+  ping();
+  setInterval(ping, 15000);
+})();
diff --git a/app/templates/base_app.html b/app/templates/base_app.html
index 4e26ad0..6dae00f 100644
--- a/app/templates/base_app.html
+++ b/app/templates/base_app.html
@@ -49,5 +49,6 @@
         {% endif %} {% endwith %} {% block content %}{% endblock %}
       </main>
     </div>
+    <script src="{{ url_for('static', filename='js/presence.js') }}"></script>
   </body>
 </html>
diff --git a/app/templates/base_public.html b/app/templates/base_public.html
index dd16f9b..9d6a689 100644
--- a/app/templates/base_public.html
+++ b/app/templates/base_public.html
@@ -20,5 +20,6 @@
       </div>
       {% endif %} {% endwith %} {% block content %}{% endblock %}
     </main>
+    <script src="{{ url_for('static', filename='js/presence.js') }}"></script>
   </body>
 </html>