diff --git a/deployment/vercel/api/lobbies.mjs b/deployment/vercel/api/lobbies.mjs
index 4e39f28..0394fe8 100644
--- a/deployment/vercel/api/lobbies.mjs
+++ b/deployment/vercel/api/lobbies.mjs
@@ -194,6 +194,13 @@ export default async function handler(request, response) {
       return response.status(200).json({ lobby });
     }
 
+    if (body.action === "leave") {
+      const lobbyId = String(body.lobbyId ?? "");
+      await callReducer(LOBBY_DATABASE, "leave_lobby", [lobbyId], token);
+      const lobby = (await listLobbies()).find((candidate) => candidate.lobbyId === lobbyId);
+      return response.status(200).json({ lobby: lobby ?? null });
+    }
+
     return response.status(400).json({ error: "unknown_action" });
   } catch (error) {
     const status = Number(error.statusCode) || 500;
diff --git a/deployment/vercel/index.html b/deployment/vercel/index.html
index 0e3de3b..24fec10 100644
--- a/deployment/vercel/index.html
+++ b/deployment/vercel/index.html
@@ -104,7 +104,7 @@
           .join("");
       }
 
-      function render(lobbies) {
+      function render(lobbies, identity) {
         list.replaceChildren();
         const visible = lobbies.filter((lobby) => !["cancelled"].includes(lobby.status));
         if (!visible.length) {
@@ -114,6 +114,7 @@
           list.append(empty);
           return;
         }
+        const self = String(identity?.identity ?? "");
         for (const lobby of visible) {
           const row = document.createElement("article");
           row.className = "lobby";
@@ -127,9 +128,30 @@
           names.className = "names";
           names.textContent = lobby.failureReason || lobby.members.map((member) => member.displayName).join(" · ");
           copy.append(title, meta, names);
+          const actions = document.createElement("div");
+          actions.style.display = "grid";
+          actions.style.gap = "8px";
+          const member = lobby.members.some((entry) => String(entry.identity) === self);
+          if (member) {
+            const leave = document.createElement("button");
+            leave.textContent = "LEAVE";
+            leave.addEventListener("click", async () => {
+              leave.disabled = true;
+              status.textContent = `Leaving ${lobby.lobbyId}…`;
+              try {
+                await api({ action: "leave", lobbyId: lobby.lobbyId });
+                status.textContent = "Left lobby.";
+                await refresh();
+              } catch (error) {
+                status.textContent = error.message;
+                leave.disabled = false;
+              }
+            });
+            actions.append(leave);
+          }
           const join = document.createElement("button");
           join.textContent = "JOIN";
-          join.disabled = lobby.status !== "open" || lobby.memberCount >= lobby.playerCount;
+          join.disabled = member || lobby.status !== "open" || lobby.memberCount >= lobby.playerCount;
           join.addEventListener("click", async () => {
             const displayName = nameInput.value.trim();
             if (!displayName) return nameInput.focus();
@@ -144,17 +166,19 @@
               join.disabled = false;
             }
           });
-          row.append(copy, join);
+          actions.append(join);
+          row.append(copy, actions);
           list.append(row);
         }
       }
 
       async function refresh() {
         try {
+          const identity = await session();
           const response = await fetch("/api/lobbies", { cache: "no-store" });
           const value = await response.json();
           if (!response.ok) throw new Error(value.error ?? "Could not load lobbies");
-          render(value.lobbies);
+          render(value.lobbies, identity);
           status.textContent = "Create a lobby or join an open front.";
         } catch (error) {
           status.textContent = error.message;
diff --git a/modules/lobby/src/lib.rs b/modules/lobby/src/lib.rs
index 245447a..22a8534 100644
--- a/modules/lobby/src/lib.rs
+++ b/modules/lobby/src/lib.rs
@@ -137,6 +137,35 @@ fn active_lobby_for_identity(ctx: &ReducerContext, identity: Identity) -> Option
         })
 }
 
+fn delete_lobby_and_members(ctx: &ReducerContext, lobby_id: &str) {
+    let member_keys = ctx
+        .db
+        .lobby_member()
+        .iter()
+        .filter(|member| member.lobby_id == lobby_id)
+        .map(|member| member.member_key)
+        .collect::<Vec<_>>();
+    for key in member_keys {
+        ctx.db.lobby_member().member_key().delete(&key);
+    }
+    ctx.db.lobby().lobby_id().delete(lobby_id.to_owned());
+}
+
+/// Pure post-leave status decision. Empty lobbies are deleted rather than kept
+/// as Failed/Cancelled rows so membership cannot block future creates.
+fn status_after_member_leave(
+    status: LobbyStatus,
+    member_count_after: u16,
+) -> Result<LobbyStatus, ()> {
+    if member_count_after == 0 {
+        return Err(());
+    }
+    Ok(match status {
+        LobbyStatus::Full => LobbyStatus::Open,
+        other => other,
+    })
+}
+
 #[spacetimedb::reducer(init)]
 pub fn init(ctx: &ReducerContext) {
     ctx.db.control_config().insert(ControlConfig {
@@ -246,6 +275,39 @@ pub fn join_lobby(
     Ok(())
 }
 
+#[spacetimedb::reducer]
+pub fn leave_lobby(ctx: &ReducerContext, lobby_id: String) -> Result<(), String> {
+    let mut lobby = ctx
+        .db
+        .lobby()
+        .lobby_id()
+        .find(&lobby_id)
+        .ok_or("lobby does not exist")?;
+    let key = member_key(&lobby_id, ctx.sender());
+    if ctx.db.lobby_member().member_key().find(&key).is_none() {
+        return Ok(());
+    }
+    ctx.db.lobby_member().member_key().delete(&key);
+
+    let member_count = lobby.member_count.saturating_sub(1);
+    match status_after_member_leave(lobby.status, member_count) {
+        Err(()) => {
+            // Last member left — including creator leaving a Pending/Provisioning
+            // lobby — so drop the row instead of leaving an orphan that blocks
+            // create_lobby via active membership.
+            delete_lobby_and_members(ctx, &lobby_id);
+            Ok(())
+        }
+        Ok(status) => {
+            lobby.member_count = member_count;
+            lobby.status = status;
+            lobby.updated_at_us = timestamp_us(ctx);
+            ctx.db.lobby().lobby_id().update(lobby);
+            Ok(())
+        }
+    }
+}
+
 #[spacetimedb::reducer]
 pub fn begin_provision(ctx: &ReducerContext, lobby_id: String) -> Result<(), String> {
     require_owner(ctx)?;
@@ -337,17 +399,7 @@ pub fn remove_inactive_lobby(ctx: &ReducerContext, lobby_id: String) -> Result<(
     if !matches!(lobby.status, LobbyStatus::Failed | LobbyStatus::Cancelled) {
         return Err("only failed or cancelled lobbies may be removed".to_owned());
     }
-    let member_keys = ctx
-        .db
-        .lobby_member()
-        .iter()
-        .filter(|member| member.lobby_id == lobby_id)
-        .map(|member| member.member_key)
-        .collect::<Vec<_>>();
-    for key in member_keys {
-        ctx.db.lobby_member().member_key().delete(&key);
-    }
-    ctx.db.lobby().lobby_id().delete(&lobby_id);
+    delete_lobby_and_members(ctx, &lobby_id);
     Ok(())
 }
 
@@ -368,4 +420,38 @@ mod tests {
         assert_eq!(clean_display_name(&"x".repeat(40)).unwrap().len(), 32);
         assert!(clean_display_name("   ").is_err());
     }
+
+    #[test]
+    fn last_member_leave_deletes_for_every_live_status() {
+        for status in [
+            LobbyStatus::Pending,
+            LobbyStatus::Provisioning,
+            LobbyStatus::Open,
+            LobbyStatus::Full,
+            LobbyStatus::Failed,
+            LobbyStatus::Cancelled,
+        ] {
+            assert_eq!(status_after_member_leave(status, 0), Err(()));
+        }
+    }
+
+    #[test]
+    fn non_empty_leave_reopens_full_lobbies_only() {
+        assert_eq!(
+            status_after_member_leave(LobbyStatus::Full, 1),
+            Ok(LobbyStatus::Open)
+        );
+        assert_eq!(
+            status_after_member_leave(LobbyStatus::Open, 1),
+            Ok(LobbyStatus::Open)
+        );
+        assert_eq!(
+            status_after_member_leave(LobbyStatus::Pending, 1),
+            Ok(LobbyStatus::Pending)
+        );
+        assert_eq!(
+            status_after_member_leave(LobbyStatus::Provisioning, 1),
+            Ok(LobbyStatus::Provisioning)
+        );
+    }
 }