diff --git a/Lib/email/_policybase.py b/Lib/email/_policybase.py
index 4b63b97217a835..9558691402d458 100644
--- a/Lib/email/_policybase.py
+++ b/Lib/email/_policybase.py
@@ -90,6 +90,14 @@ def __add__(self, other):
         """
         return self.clone(**other.__dict__)
 
+def validate_header(name):
+    # Validate header name according to RFC 5322
+    import re
+    if not re.match(r'^[^\s:]+$', name):
+        raise ValueError(f"Invalid header field name {name!r}")
+    # Only allow printable ASCII characters
+    if any(ord(c) < 33 or ord(c) > 126 for c in name):
+        raise ValueError(f"Invalid header field name {name!r}")
 
 def _append_doc(doc, added_doc):
     doc = doc.rsplit('\n', 1)[0]
@@ -314,6 +322,7 @@ def header_store_parse(self, name, value):
         """+
         The name and value are returned unmodified.
         """
+        validate_header(name)
         return (name, value)
 
     def header_fetch_parse(self, name, value):
diff --git a/Lib/email/policy.py b/Lib/email/policy.py
index 6e109b65011a44..48e6b2340e2c61 100644
--- a/Lib/email/policy.py
+++ b/Lib/email/policy.py
@@ -4,7 +4,7 @@
 
 import re
 import sys
-from email._policybase import Policy, Compat32, compat32, _extend_docstrings
+from email._policybase import Policy, Compat32, compat32, _extend_docstrings, validate_header
 from email.utils import _has_surrogates
 from email.headerregistry import HeaderRegistry as HeaderRegistry
 from email.contentmanager import raw_data_manager
@@ -140,6 +140,7 @@ def header_store_parse(self, name, value):
         """
         if hasattr(value, 'name') and value.name.lower() == name.lower():
             return (name, value)
+        validate_header(name)
         if isinstance(value, str) and len(value.splitlines())>1:
             # XXX this error message isn't quite right when we use splitlines
             # (see issue 22233), but I'm not sure what should happen here.
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index abe9ef2e94409f..ade9d3a2ac28f5 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -728,6 +728,29 @@ def test_nonascii_add_header_with_tspecial(self):
             "attachment; filename*=utf-8''Fu%C3%9Fballer%20%5Bfilename%5D.ppt",
             msg['Content-Disposition'])
 
+    def test_invalid_headers(self):
+        invalid_headers = [
+            ('Invalid Header', 'contains space'),
+            ('Tab\tHeader', 'contains tab'),
+            ('Colon:Header', 'contains colon'),
+            ('', 'Empty name'),
+            (' LeadingSpace', 'starts with space'),
+            ('TrailingSpace ', 'ends with space'),
+        ]
+        for name, value in invalid_headers:
+            with self.assertRaises(ValueError) as cm:
+               Message().add_header(name, value)
+            self.assertIn(f"Invalid header field name {name!r}", str(cm.exception))
+
+        invalid_headers = [
+            ('Header\x7F', 'Non-ASCII character'),
+            ('Header\x1F', 'control character'),
+        ]
+        for name, value in invalid_headers:
+            with self.assertRaises(ValueError) as cm:
+               Message().add_header(name, value)
+            self.assertIn(f"Invalid header field name {name!r}", str(cm.exception))
+
     def test_binary_quopri_payload(self):
         for charset in ('latin-1', 'ascii'):
             msg = Message()
diff --git a/Misc/NEWS.d/next/Library/2024-12-11-17-44-36.gh-issue-127794.VwmRsp.rst b/Misc/NEWS.d/next/Library/2024-12-11-17-44-36.gh-issue-127794.VwmRsp.rst
new file mode 100644
index 00000000000000..c5ed88bd2bf61b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-12-11-17-44-36.gh-issue-127794.VwmRsp.rst
@@ -0,0 +1,2 @@
+The :meth:`email.message.Message.add_header` method now validates header
+field names according to :rfc:`5322`.