diff --git a/keyring/backend.py b/keyring/backend.py
index 7680e4b3..82c8410b 100644
--- a/keyring/backend.py
+++ b/keyring/backend.py
@@ -6,10 +6,12 @@
 
 import abc
 import copy
+import functools
 import logging
 import operator
 import os
 import typing
+import warnings
 
 from jaraco.functools import once
 
@@ -26,18 +28,38 @@
 
 class KeyringBackendMeta(abc.ABCMeta):
     """
-    A metaclass that's both an ABCMeta and a type that keeps a registry of
-    all (non-abstract) types.
+    Specialized subclass behavior.
+
+    Keeps a registry of all (non-abstract) types.
+
+    Wraps set_password to validate the username.
     """
 
     def __init__(cls, name, bases, dict):
         super().__init__(name, bases, dict)
+        cls._register()
+        cls._validate_username_in_set_password()
+
+    def _register(cls):
         if not hasattr(cls, '_classes'):
             cls._classes = set()
         classes = cls._classes
         if not cls.__abstractmethods__:
             classes.add(cls)
 
+    def _validate_username_in_set_password(cls):
+        """
+        Wrap ``set_password`` such to validate the passed username.
+        """
+        orig = cls.set_password
+
+        @functools.wraps(orig)
+        def wrapper(self, system, username, *args, **kwargs):
+            self._validate_username(username)
+            return orig(self, system, username, *args, **kwargs)
+
+        cls.set_password = wrapper
+
 
 class KeyringBackend(metaclass=KeyringBackendMeta):
     """The abstract base class of the keyring, every backend must implement
@@ -100,6 +122,18 @@ def get_password(self, service: str, username: str) -> str | None:
         """Get password of the username for the service"""
         return None
 
+    def _validate_username(self, username: str) -> None:
+        """
+        Ensure the username is not empty.
+        """
+        if not username:
+            warnings.warn(
+                "Empty usernames are deprecated. See #668",
+                DeprecationWarning,
+                stacklevel=3,
+            )
+            # raise ValueError("Username cannot be empty")
+
     @abc.abstractmethod
     def set_password(self, service: str, username: str, password: str) -> None:
         """Set password for the username of the service.
diff --git a/keyring/testing/backend.py b/keyring/testing/backend.py
index ac9ad6de..45b0b2aa 100644
--- a/keyring/testing/backend.py
+++ b/keyring/testing/backend.py
@@ -163,6 +163,12 @@ def test_credential(self):
             ('user2', 'password2'),
         )
 
+    @pytest.mark.xfail("platform.system() == 'Windows'", reason="#668")
+    def test_empty_username(self):
+        with pytest.deprecated_call():
+            self.set_password('service1', '', 'password1')
+        assert self.keyring.get_password('service1', '') == 'password1'
+
     def test_set_properties(self, monkeypatch):
         env = dict(KEYRING_PROPERTY_FOO_BAR='fizz buzz', OTHER_SETTING='ignore me')
         monkeypatch.setattr(os, 'environ', env)
diff --git a/newsfragments/668.feature.rst b/newsfragments/668.feature.rst
new file mode 100644
index 00000000..f4b98274
--- /dev/null
+++ b/newsfragments/668.feature.rst
@@ -0,0 +1 @@
+Deprecated support for empty usernames. Now all backends will reject an empty string as input for the 'username' field when setting a password. Later this deprecation will become a more visible user warning and even later an error. If this warning is triggered in your environment, please consider using a static value (even 'username') or comment in the issue and describe the use-case that demands support for empty usernames.
\ No newline at end of file