diff --git a/AUTHORS b/AUTHORS
index a089ca678f7..ff37c8fbf26 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -311,6 +311,7 @@ Michael Goerz
 Michael Krebs
 Michael Seifert
 Michael Vogt
+Michael Reznik
 Michal Wajszczuk
 Michał Górny
 Michał Zięba
diff --git a/changelog/13634.bugfix.rst b/changelog/13634.bugfix.rst
new file mode 100644
index 00000000000..ee12aeafc3a
--- /dev/null
+++ b/changelog/13634.bugfix.rst
@@ -0,0 +1,5 @@
+Blocking a ``conftest.py`` file using the ``-p no:`` option is now explicitly disallowed.
+
+Previously this resulted in an internal assertion failure during plugin loading.
+
+Pytest now raises a clear ``UsageError`` explaining that conftest files are not plugins and cannot be disabled via ``-p``.
diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index 51c99acd8e6..21dc35219d8 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -816,6 +816,12 @@ def consider_pluginarg(self, arg: str) -> None:
             if name in essential_plugins:
                 raise UsageError(f"plugin {name} cannot be disabled")
 
+            if name.endswith("conftest.py"):
+                raise UsageError(
+                    f"Blocking conftest files using -p is not supported: -p no:{name}\n"
+                    "conftest.py files are not plugins and cannot be disabled via -p.\n"
+                )
+
             # PR #4304: remove stepwise if cacheprovider is blocked.
             if name == "cacheprovider":
                 self.set_blocked("stepwise")
diff --git a/testing/test_config.py b/testing/test_config.py
index cb916a8b15a..f6598ff2508 100644
--- a/testing/test_config.py
+++ b/testing/test_config.py
@@ -2460,6 +2460,10 @@ def test_config_does_not_load_blocked_plugin_from_args(pytester: Pytester) -> No
     result.stderr.fnmatch_lines(["*: error: unrecognized arguments: -s"])
     assert result.ret == ExitCode.USAGE_ERROR
 
+    result = pytester.runpytest(str(p), "-p no:/path/to/conftest.py", "-s")
+    result.stderr.fnmatch_lines(["ERROR:*Blocking conftest files*"])
+    assert result.ret == ExitCode.USAGE_ERROR
+
 
 def test_invocation_args(pytester: Pytester) -> None:
     """Ensure that Config.invocation_* arguments are correctly defined"""