diff --git a/docs/changelog/2658.feature.rst b/docs/changelog/2658.feature.rst
new file mode 100644
index 000000000..92dcaf752
--- /dev/null
+++ b/docs/changelog/2658.feature.rst
@@ -0,0 +1 @@
+Fail on :ref:`pass_env`/:ref:`passenv` entries containing whitespace - by :user:`ericzolf`.
diff --git a/src/tox/tox_env/api.py b/src/tox/tox_env/api.py
index abcdc78b4..da042f523 100644
--- a/src/tox/tox_env/api.py
+++ b/src/tox/tox_env/api.py
@@ -7,6 +7,7 @@
 import logging
 import os
 import re
+import string
 import sys
 from abc import ABC, abstractmethod
 from contextlib import contextmanager
@@ -21,7 +22,7 @@
 from tox.execute.request import ExecuteRequest
 from tox.journal import EnvJournal
 from tox.report import OutErr, ToxHandler
-from tox.tox_env.errors import Recreate, Skip
+from tox.tox_env.errors import Fail, Recreate, Skip
 from tox.tox_env.info import Info
 from tox.tox_env.installer import Installer
 from tox.util.path import ensure_empty_dir
@@ -132,7 +133,16 @@ def register_config(self) -> None:
 
         def pass_env_post_process(values: list[str]) -> list[str]:
             values.extend(self._default_pass_env())
-            return sorted({k: None for k in values}.keys())
+            result = sorted({k: None for k in values}.keys())
+            invalid_chars = set(string.whitespace)
+            invalid = [v for v in result if any(c in invalid_chars for c in v)]
+            if invalid:
+                invalid_repr = ", ".join(repr(i) for i in invalid)
+                raise Fail(
+                    f"pass_env values cannot contain whitespace, use comma to have multiple values in a single line, "
+                    f"invalid values found {invalid_repr}",
+                )
+            return result
 
         self.conf.add_config(
             keys=["pass_env", "passenv"],
diff --git a/tests/tox_env/test_tox_env_api.py b/tests/tox_env/test_tox_env_api.py
index 2c467bb6b..700974430 100644
--- a/tests/tox_env/test_tox_env_api.py
+++ b/tests/tox_env/test_tox_env_api.py
@@ -80,6 +80,26 @@ def test_tox_env_pass_env_literal_miss() -> None:
     assert not env
 
 
+def test_tox_env_pass_env_fails_on_whitespace(tox_project: ToxProjectCreator) -> None:
+    first, second = "A B", "C D"
+    prj = tox_project({"tox.ini": f"[testenv]\npackage=skip\npass_env = {first}\n {second}\n  E"})
+    result = prj.run("c", "-k", "pass_env")
+    result.assert_success()
+    msg = (
+        '[testenv:py]\npass_env = # Exception: Fail("pass_env values cannot contain whitespace, use comma to have '
+        f'multiple values in a single line, invalid values found {first!r}, {second!r}")\n\n[tox]\n'
+    )
+    assert result.out == msg
+
+    result = prj.run("r")
+    result.assert_failed(1)
+    msg = (
+        "py: failed with pass_env values cannot contain whitespace, use comma to have multiple values in a single line,"
+        " invalid values found 'A B', 'C D'"
+    )
+    assert msg in result.out
+
+
 @pytest.mark.parametrize("glob", ["*", "?"])
 @pytest.mark.parametrize("char", ["a", "A"])
 def test_tox_env_pass_env_match_ignore_case(char: str, glob: str) -> None: