diff --git a/cli/src/commands/git.rs b/cli/src/commands/git.rs
index 0550ec513..fdf4ec382 100644
--- a/cli/src/commands/git.rs
+++ b/cli/src/commands/git.rs
@@ -441,7 +441,7 @@ fn cmd_git_clone(
         }
     }
 
-    if let (mut workspace_command, Some(default_branch)) = clone_result? {
+    if let (mut workspace_command, git_repo, Some(default_branch)) = clone_result? {
         let default_branch_target = workspace_command
             .repo()
             .view()
@@ -449,6 +449,12 @@ fn cmd_git_clone(
         if let Some(commit_id) = default_branch_target.as_normal().cloned() {
             let mut checkout_tx =
                 workspace_command.start_transaction("check out git remote's default branch");
+            if args.colocate {
+                // HEAD can't be detached without specifying a commit, so we set the new default
+                // branch instead. Otherwise, the old "unborn" default branch would move
+                // alongside the new default branch.
+                git_repo.set_head(&format!("refs/heads/{default_branch}"))?;
+            }
             if let Ok(commit) = checkout_tx.repo().store().get_commit(&commit_id) {
                 checkout_tx.check_out(&commit)?;
             }
@@ -464,7 +470,7 @@ fn do_git_clone(
     colocate: bool,
     source: &str,
     wc_path: &Path,
-) -> Result<(WorkspaceCommandHelper, Option<String>), CommandError> {
+) -> Result<(WorkspaceCommandHelper, git2::Repository, Option<String>), CommandError> {
     let (workspace, repo) = if colocate {
         let git_repo = git2::Repository::init(wc_path)?;
         Workspace::init_external_git(command.settings(), wc_path, git_repo.path())?
@@ -499,7 +505,7 @@ fn do_git_clone(
         }
     })?;
     fetch_tx.finish(ui)?;
-    Ok((workspace_command, maybe_default_branch))
+    Ok((workspace_command, git_repo, maybe_default_branch))
 }
 
 fn with_remote_callbacks<T>(ui: &mut Ui, f: impl FnOnce(git::RemoteCallbacks<'_>) -> T) -> T {
diff --git a/cli/tests/test_git_clone.rs b/cli/tests/test_git_clone.rs
index 32a0e1c40..b233ee778 100644
--- a/cli/tests/test_git_clone.rs
+++ b/cli/tests/test_git_clone.rs
@@ -198,6 +198,12 @@ fn test_git_clone_colocate() {
             .symbolic_target()
     );
 
+    // The old default branch "master" shouldn't exist.
+    let stdout = test_env.jj_cmd_success(&test_env.env_root().join("clone"), &["branch", "list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    main: mzyxwzks 9f01a0e0 message
+    "###);
+
     // Subsequent fetch should just work even if the source path was relative
     let stdout = test_env.jj_cmd_success(&test_env.env_root().join("clone"), &["git", "fetch"]);
     insta::assert_snapshot!(stdout, @r###"