diff --git a/src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs b/src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs
index 835ab22793730c..4b889b8aba92e0 100644
--- a/src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs
@@ -83,7 +83,10 @@ public DirectoryInfo CreateSubdirectory(string path)
             string newPath = Path.GetFullPath(Path.Combine(FullPath, path));
 
             ReadOnlySpan<char> trimmedNewPath = Path.TrimEndingDirectorySeparator(newPath.AsSpan());
-            ReadOnlySpan<char> trimmedCurrentPath = Path.TrimEndingDirectorySeparator(FullPath.AsSpan());
+
+            // Trim any trailing separator, including from a root path for proper boundary checking.
+            // TrimEndingDirectorySeparator does not trim the character from path roots.
+            ReadOnlySpan<char> trimmedCurrentPath = FullPath.TrimEnd(Path.DirectorySeparatorChar);
 
             // We want to make sure the requested directory is actually under the subdirectory.
             if (trimmedNewPath.StartsWith(trimmedCurrentPath, PathInternal.StringComparison)
diff --git a/src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DirectoryInfo/CreateSubdirectory.cs b/src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DirectoryInfo/CreateSubdirectory.cs
index 0878b396b0983e..f16bd0c1617f73 100644
--- a/src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DirectoryInfo/CreateSubdirectory.cs
+++ b/src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DirectoryInfo/CreateSubdirectory.cs
@@ -218,6 +218,41 @@ public void ExtendedPathSubdirectory()
             Assert.StartsWith(IOInputs.ExtendedPrefix, subDir.FullName);
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void CreateSubdirectory_RootDriveSubfolder_Windows()
+        {
+            string rootDrive = Path.GetPathRoot(Environment.SystemDirectory);
+            DirectoryInfo rootDirectory = new DirectoryInfo(rootDrive);
+            string testFolderName = GetTestFileName();
+
+            try
+            {
+                DirectoryInfo subDirectory = rootDirectory.CreateSubdirectory(testFolderName);
+
+                Assert.True(subDirectory.Exists);
+                Assert.Equal(Path.Combine(rootDrive, testFolderName), subDirectory.FullName);
+            }
+            finally
+            {
+                string testFolderPath = Path.Combine(rootDrive, testFolderName);
+                if (Directory.Exists(testFolderPath))
+                {
+                    Directory.Delete(testFolderPath, recursive: true);
+                }
+            }
+        }
+
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Linux)]
+        public void CreateSubdirectory_RootDriveSubfolder_ThrowsUnauthorizedAccessException_Linux()
+        {
+            DirectoryInfo rootDirectory = new DirectoryInfo("/");
+            string testFolderName = GetTestFileName();
+
+            Assert.Throws<UnauthorizedAccessException>(() => rootDirectory.CreateSubdirectory(testFolderName));
+        }
+
         [Fact]
         [PlatformSpecific(TestPlatforms.Windows)] // UNC shares
         public void UNCPathWithOnlySlashes()