diff --git a/gin/config.py b/gin/config.py
index 62bf3fc..836db58 100644
--- a/gin/config.py
+++ b/gin/config.py
@@ -2797,7 +2797,7 @@ def constant(name, value):
   if not config_parser.MODULE_RE.match(name):
     raise ValueError("Invalid constant selector '{}'.".format(name))
 
-  if _CONSTANTS.matching_selectors(name):
+  if not _INTERACTIVE_MODE and _CONSTANTS.matching_selectors(name):
     err_str = "Constants matching selector '{}' already exist ({})."
     raise ValueError(err_str.format(name, _CONSTANTS.matching_selectors(name)))
 
diff --git a/tests/config_test.py b/tests/config_test.py
index 4622ddc..44c8c3e 100644
--- a/tests/config_test.py
+++ b/tests/config_test.py
@@ -2114,6 +2114,20 @@ def duplicate_fn4():  # pylint: disable=unused-variable
     config.parse_config(config_str)
     self.assertEqual(ConfigurableClass().kwarg1, 'duplicate_fn3')
 
+    config.constant('CONST1', 42)
+    with self.assertRaisesRegex(ValueError, 'already exist'):
+      config.constant('CONST1', 42)
+
+    with config.interactive_mode():
+      config.constant('CONST1', 43)
+
+    config_str = """
+      configurable2.non_kwarg = %CONST1
+    """
+    config.parse_config(config_str)
+    non_kwarg, _ = configurable2()  # pylint:disable=no-value-for-parameter
+    self.assertIs(non_kwarg, 43)
+
   def testFinalizeLocksConfig(self):
     config.finalize()
     with self.assertRaises(RuntimeError):