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