diff --git a/babel/core.py b/babel/core.py
index 3ba8b85ea..d173e4863 100644
--- a/babel/core.py
+++ b/babel/core.py
@@ -361,6 +361,12 @@ def parse(
         if not isinstance(identifier, str):
             raise TypeError(f"Unexpected value for identifier: {identifier!r}")
 
+        # C/POSIX is not a CLDR language. Same mapping default_locale uses.
+        posix_stem = identifier.split(".")[0].split("@")[0]
+        posix_stem = posix_stem.replace("-", "_")
+        if posix_stem.upper() in {"C", "POSIX"}:
+            identifier = "en_US_POSIX"
+
         parts = parse_locale(identifier, sep=sep)
         input_id = get_locale_identifier(parts)
 
diff --git a/tests/test_core.py b/tests/test_core.py
index 2c1e6d92e..445ed67a8 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -56,6 +56,12 @@ def test_default_locale(monkeypatch):
         assert default_locale() == 'en_US_POSIX'
 
 
+def test_locale_parse_accepts_c_and_posix():
+    expected = Locale.parse('en_US_POSIX')
+    for value in ['C', 'c', 'POSIX', 'posix', 'C.UTF-8', 'C.utf8', 'POSIX.UTF-8']:
+        assert Locale.parse(value) == expected
+
+
 def test_default_locale_multiple_args(monkeypatch):
     for name in [
         'LANGUAGE',