diff --git a/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ResourceDictionary.cs b/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ResourceDictionary.cs
index 84e2c746bdf..02ebd9fe50a 100644
--- a/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ResourceDictionary.cs
+++ b/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ResourceDictionary.cs
@@ -1508,6 +1508,10 @@ internal void AddOwner(DispatcherObject owner)
                     fe.ShouldLookupImplicitStyles = true;
                 }
 
+                // Owned dictionaries are never theme dictionaries; clear the flag the ctor may
+                // have inherited from SystemResources.IsSystemResourcesParsing. Propagates to merged dictionaries.
+                IsThemeDictionary = false;
+
                 _ownerFEs.Add(fe);
             }
             else
@@ -1530,6 +1534,10 @@ internal void AddOwner(DispatcherObject owner)
                         fce.ShouldLookupImplicitStyles = true;
                     }
 
+                    // Owned dictionaries are never theme dictionaries; clear the flag the ctor may
+                    // have inherited from SystemResources.IsSystemResourcesParsing. Propagates to merged dictionaries.
+                    IsThemeDictionary = false;
+
                     _ownerFCEs.Add(fce);
                 }
                 else
@@ -1557,6 +1565,10 @@ internal void AddOwner(DispatcherObject owner)
                         // An Application ResourceDictionary can be accessed across threads
                         CanBeAccessedAcrossThreads = true;
 
+                        // Owned dictionaries are never theme dictionaries; clear the flag the ctor may
+                        // have inherited from SystemResources.IsSystemResourcesParsing. Propagates to merged dictionaries.
+                        IsThemeDictionary = false;
+
                         // Seal all the styles and templates in this app dictionary
                         SealValues();
                     }
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/PresentationFramework.Tests/System/Windows/ResourceDictionaryTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/PresentationFramework.Tests/System/Windows/ResourceDictionaryTests.cs
index 492da8764c5..2af54568a1b 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/PresentationFramework.Tests/System/Windows/ResourceDictionaryTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/PresentationFramework.Tests/System/Windows/ResourceDictionaryTests.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Linq;
+using System.Reflection;
 using System.Windows.Controls;
 using System.Windows.Media;
 using System.Windows.Threading;
@@ -159,4 +160,19 @@ public void DynamicResource_ShouldRemainLastValue_WhenLocalResourceRemoved()
         // The default foreground color of textBlock (as in aero2)
         textBlock.Foreground.Should().BeSameAs(Brushes.Black);
     }
+
+    [WpfFact]
+    public void AddOwner_ClearsInheritedIsThemeDictionaryFlag()
+    {
+        // Simulate a ResourceDictionary whose ctor inherited
+        // IsThemeDictionary from SystemResources.IsSystemResourcesParsing.
+        PropertyInfo isThemeDictionary = typeof(ResourceDictionary).GetProperty(
+            "IsThemeDictionary", BindingFlags.Instance | BindingFlags.NonPublic)!;
+        var rd = new ResourceDictionary();
+        isThemeDictionary.SetValue(rd, true);
+
+        _ = new Button { Resources = rd };
+
+        isThemeDictionary.GetValue(rd).Should().Be(false);
+    }
 }