diff --git a/src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs b/src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs
index 6896e2614b1c2..4cc4f8b120fd9 100644
--- a/src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs
+++ b/src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs
@@ -442,9 +442,8 @@ private static bool IsChildPath(string parentFullPath, string childFullPath)
 
             return childFullPath.StartsWith(parentFullPath, PathInternal.StringComparison);
 
-            // We don't need to check for AltDirectorySeparatorChar, full paths are normalized to DirectorySeparatorChar.
             static bool IsDirectorySeparatorChar(char c)
-                => c == Path.DirectorySeparatorChar;
+                => c == Path.DirectorySeparatorChar || c == Path.AltDirectorySeparatorChar;
         }
     }
 }