diff --git a/docs/design/companion-process-manager.md b/docs/design/companion-process-manager.md
index 7b11a176..e2bbfe70 100644
--- a/docs/design/companion-process-manager.md
+++ b/docs/design/companion-process-manager.md
@@ -675,7 +675,7 @@ No per-companion logic in Arbiter.
 - [x] Apply `cwd` and `env` before target.
 - [x] Redirect `stdout` and `stderr`.
 - [x] Reap exited companion processes.
-- [ ] Implement `STARTING -> RUNNING` using `startsecs`.
+- [x] Implement `STARTING -> RUNNING` using `startsecs`.
 - [ ] Implement `BACKOFF` with fixed `companion_restart_delay`.
 - [ ] Implement `start_process`.
 - [ ] Implement `stop_process`.
diff --git a/gunicorn/companion/manager.py b/gunicorn/companion/manager.py
index 413c46a2..36ef60c5 100644
--- a/gunicorn/companion/manager.py
+++ b/gunicorn/companion/manager.py
@@ -84,6 +84,24 @@ class CompanionManager:
                 reaped.append(proc)
         return reaped
 
+    def promote_running(self, now: float = None) -> list:
+        """Move companions that survived ``startsecs`` from STARTING to RUNNING.
+
+        A freshly spawned companion starts in STARTING. If it stays alive for
+        its ``startsecs`` window it is considered up and becomes RUNNING; if it
+        dies first, reaping handles it instead. Returns the promoted ones.
+        """
+        now = now or time.time()
+        promoted = []
+        for proc in self.processes.values():
+            if proc.state != State.STARTING or proc.started_at is None:
+                continue
+            if now - proc.started_at >= proc.config.startsecs:
+                proc.state = State.RUNNING
+                self.log.info("companion %s running (pid %s)", proc.name, proc.pid)
+                promoted.append(proc)
+        return promoted
+
     def _process_by_pid(self, pid: int):
         for proc in self.processes.values():
             if proc.pid == pid:
diff --git a/tests/test_companion_manager.py b/tests/test_companion_manager.py
index 0169b8cf..c1c38826 100644
--- a/tests/test_companion_manager.py
+++ b/tests/test_companion_manager.py
@@ -125,6 +125,36 @@ def test_reap_no_children():
         assert mgr.reap_processes() == []
 
 
+def test_promote_running_after_startsecs():
+    mgr = make_manager("rq")
+    proc = mgr.processes["rq"]
+    proc.config.startsecs = 1
+    proc.state = State.STARTING
+    proc.started_at = 100.0
+    promoted = mgr.promote_running(now=101.5)
+    assert promoted == [proc]
+    assert proc.state == State.RUNNING
+
+
+def test_promote_running_too_early():
+    mgr = make_manager("rq")
+    proc = mgr.processes["rq"]
+    proc.config.startsecs = 5
+    proc.state = State.STARTING
+    proc.started_at = 100.0
+    assert mgr.promote_running(now=102.0) == []
+    assert proc.state == State.STARTING
+
+
+def test_promote_running_ignores_non_starting():
+    mgr = make_manager("rq")
+    proc = mgr.processes["rq"]
+    proc.state = State.BACKOFF
+    proc.started_at = 100.0
+    assert mgr.promote_running(now=999.0) == []
+    assert proc.state == State.BACKOFF
+
+
 def test_spawn_parent_records_pid_and_starting():
     mgr = make_manager("rq")
     proc = mgr.processes["rq"]