diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index ad65ed29929..fa8d5457f1c 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -40,6 +40,14 @@ class ExitCode(enum.IntEnum):
     #: pytest couldn't find tests
     NO_TESTS_COLLECTED = 5
 
+    @classmethod
+    def _missing_(cls, value):
+        pseudo_member = int.__new__(cls, value)
+        pseudo_member._name_ = "CUSTOM"
+        pseudo_member._value_ = value
+        cls._value2member_map_[value] = pseudo_member
+        return pseudo_member
+
 
 def pytest_addoption(parser):
     parser.addini(
diff --git a/testing/test_main.py b/testing/test_main.py
new file mode 100644
index 00000000000..0450bcda9cc
--- /dev/null
+++ b/testing/test_main.py
@@ -0,0 +1,11 @@
+from _pytest.main import ExitCode
+
+
+def test_ExitCode():
+    exc_0 = ExitCode(0)
+    assert exc_0 == 0
+    assert repr(exc_0) == "<ExitCode.OK: 0>"
+
+    exc_99 = ExitCode(99)
+    assert exc_99 == 99
+    assert repr(exc_99) == "<ExitCode.CUSTOM: 99>"