diff --git a/nox/sessions.py b/nox/sessions.py
index a71b6789..a1693966 100644
--- a/nox/sessions.py
+++ b/nox/sessions.py
@@ -202,7 +202,7 @@ def create_tmp(self) -> str:
         """Create, and return, a temporary directory."""
         tmpdir = os.path.join(self._runner.envdir, "tmp")
         os.makedirs(tmpdir, exist_ok=True)
-        self.env["TMPDIR"] = tmpdir
+        self.env["TMPDIR"] = os.path.abspath(tmpdir)
         return tmpdir
 
     @property
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 262d9f32..a90aa687 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -84,7 +84,7 @@ def test_create_tmp(self):
         with tempfile.TemporaryDirectory() as root:
             runner.global_config.envdir = root
             tmpdir = session.create_tmp()
-            assert session.env["TMPDIR"] == tmpdir
+            assert session.env["TMPDIR"] == os.path.abspath(tmpdir)
             assert tmpdir.startswith(root)
 
     def test_create_tmp_twice(self):
@@ -94,7 +94,7 @@ def test_create_tmp_twice(self):
             runner.venv.bin = bin
             session.create_tmp()
             tmpdir = session.create_tmp()
-            assert session.env["TMPDIR"] == tmpdir
+            assert session.env["TMPDIR"] == os.path.abspath(tmpdir)
             assert tmpdir.startswith(root)
 
     def test_properties(self):