diff --git a/docs/changelog/2449.bugfix.rst b/docs/changelog/2449.bugfix.rst
new file mode 100644
index 000000000..8fbf6790d
--- /dev/null
+++ b/docs/changelog/2449.bugfix.rst
@@ -0,0 +1 @@
+Fix ``isolated_build`` when the build process produces stderr at exit.
diff --git a/src/tox/package/builder/isolated.py b/src/tox/package/builder/isolated.py
index 1b3f6f491..00ef8f20f 100644
--- a/src/tox/package/builder/isolated.py
+++ b/src/tox/package/builder/isolated.py
@@ -125,6 +125,7 @@ def perform_isolated_build(build_info, package_venv, dist_dir, setup_dir):
                 os.path.pathsep.join(str(p) for p in build_info.backend_paths),
             ],
             returnout=True,
+            capture_err=False,
             action=action,
             cwd=setup_dir,
         )