diff --git a/python/ray/_private/parameter.py b/python/ray/_private/parameter.py
index ec61cff7702d..a05230f11bf7 100644
--- a/python/ray/_private/parameter.py
+++ b/python/ray/_private/parameter.py
@@ -96,7 +96,7 @@ class RayParams:
         raylet_socket_name: If provided, it will specify the socket path
             used by the raylet process.
         temp_dir: If provided, it will specify the root temporary
-            directory for the Ray process.
+            directory for the Ray process. Must be an absolute path.
         storage: Specify a URI for persistent cluster-wide storage. This storage path
             must be accessible by all nodes of the cluster, otherwise an error will be
             raised.
@@ -428,6 +428,9 @@ def build_error(resource, alternative):
                 "serialization. Upgrade numpy if using with ray."
             )
 
+        if self.temp_dir is not None and not os.path.isabs(self.temp_dir):
+            raise ValueError("temp_dir must be absolute path or None.")
+
     def _format_ports(self, pre_selected_ports):
         """Format the pre-selected ports information to be more human-readable."""
         ports = pre_selected_ports.copy()
diff --git a/python/ray/_private/worker.py b/python/ray/_private/worker.py
index 9f5570a88b9d..e53143710133 100644
--- a/python/ray/_private/worker.py
+++ b/python/ray/_private/worker.py
@@ -1240,8 +1240,8 @@ def init(
         _redis_password: Prevents external clients without the password
             from connecting to Redis if provided.
         _temp_dir: If provided, specifies the root temporary
-            directory for the Ray process. Defaults to an OS-specific
-            conventional location, e.g., "/tmp/ray".
+            directory for the Ray process. Must be an absolute path. Defaults to an
+            OS-specific conventional location, e.g., "/tmp/ray".
         _metrics_export_port: Port number Ray exposes system metrics
             through a Prometheus endpoint. It is currently under active
             development, and the API is subject to change.
diff --git a/python/ray/tests/test_ray_init_2.py b/python/ray/tests/test_ray_init_2.py
index 3f37dd701308..cd3e3f95f69c 100644
--- a/python/ray/tests/test_ray_init_2.py
+++ b/python/ray/tests/test_ray_init_2.py
@@ -325,6 +325,13 @@ def test_get_ray_address_from_environment(monkeypatch):
     )
 
 
+# https://github.com/ray-project/ray/issues/36431
+def test_temp_dir_must_be_absolute(shutdown_only):
+    # This test fails with a relative path _temp_dir.
+    with pytest.raises(ValueError):
+        ray.init(_temp_dir="relative_path")
+
+
 if __name__ == "__main__":
     import sys