diff --git a/src/libraries/Common/src/Interop/Interop.TimeZoneInfo.cs b/src/libraries/Common/src/Interop/Interop.TimeZoneInfo.cs
index 10e73ee4b1d791..72db59c74402c5 100644
--- a/src/libraries/Common/src/Interop/Interop.TimeZoneInfo.cs
+++ b/src/libraries/Common/src/Interop/Interop.TimeZoneInfo.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Runtime.InteropServices;
 
 internal static partial class Interop
@@ -16,7 +17,7 @@ internal static extern unsafe ResultCode GetTimeZoneDisplayName(
             int resultLength);
 
         [DllImport(Libraries.GlobalizationNative, CharSet = CharSet.Unicode, EntryPoint = "GlobalizationNative_WindowsIdToIanaId")]
-        internal static extern unsafe int WindowsIdToIanaId(string windowsId, [MarshalAs(UnmanagedType.LPStr)] string? region, char* ianaId, int ianaIdLength);
+        internal static extern unsafe int WindowsIdToIanaId(string windowsId, IntPtr region, char* ianaId, int ianaIdLength);
 
         [DllImport(Libraries.GlobalizationNative, CharSet = CharSet.Unicode, EntryPoint = "GlobalizationNative_IanaIdToWindowsId")]
         internal static extern unsafe int IanaIdToWindowsId(string ianaId, char* windowsId, int windowsIdLength);
diff --git a/src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.FullGlobalizationData.cs b/src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.FullGlobalizationData.cs
index 557e333d4eb9a5..e47060d7e6aa0c 100644
--- a/src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.FullGlobalizationData.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.FullGlobalizationData.cs
@@ -65,8 +65,34 @@ private static unsafe bool TryConvertWindowsIdToIanaId(string windowsId, string?
                 }
             }
 
+            // regionPtr will point at the region name encoded as ASCII.
+            IntPtr regionPtr = IntPtr.Zero;
+
+             // Regions usually are 2 or 3 characters length.
+            const int MaxRegionNameLength = 11;
+
+            // Ensure uppercasing the region as ICU require the region names be uppercased, otherwise ICU will assume default region and return unexpected result.
+            if (region is not null && region.Length < MaxRegionNameLength)
+            {
+                byte* regionInAscii = stackalloc byte[region.Length + 1];
+                int i = 0;
+                for (; i < region.Length && region[i] <= '\u007F'; i++)
+                {
+                    regionInAscii[i] = (uint)(region[i] - 'a') <= ('z' - 'a') ? (byte)((region[i] - 'a') + 'A') : (byte)region[i];
+                }
+
+                if (i >= region.Length)
+                {
+                    regionInAscii[region.Length] = 0;
+                    regionPtr = new IntPtr(regionInAscii);
+                }
+
+                // In case getting unexpected region names, we just fallback using the default region (pasing null region name to the ICU API).
+            }
+
             char* buffer = stackalloc char[100];
-            int length = Interop.Globalization.WindowsIdToIanaId(windowsId, region, buffer, 100);
+
+            int length = Interop.Globalization.WindowsIdToIanaId(windowsId, regionPtr, buffer, 100);
             if (length > 0)
             {
                 ianaId = allocate ? new string(buffer, 0, length) : null;
diff --git a/src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs b/src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs
index 9faa4715690373..38c8bd8c3393a0 100644
--- a/src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs
+++ b/src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs
@@ -2696,6 +2696,19 @@ public static void IdsConversionsTest(string windowsId, string ianaId)
         [InlineData("Central Europe Standard Time", "Europe/Tirane", "AL")]
         [InlineData("Central Europe Standard Time", "Europe/Podgorica", "ME")]
         [InlineData("Central Europe Standard Time", "Europe/Belgrade", "RS")]
+        // lowercased region name cases:
+        [InlineData("Cen. Australia Standard Time", "Australia/Adelaide", "au")]
+        [InlineData("AUS Central Standard Time", "Australia/Darwin", "au")]
+        [InlineData("E. Australia Standard Time", "Australia/Brisbane", "au")]
+        [InlineData("AUS Eastern Standard Time", "Australia/Sydney", "au")]
+        [InlineData("Tasmania Standard Time", "Australia/Hobart", "au")]
+        [InlineData("Romance Standard Time", "Europe/Madrid", "es")]
+        [InlineData("Romance Standard Time", "Europe/Madrid", "Es")]
+        [InlineData("Romance Standard Time", "Europe/Madrid", "eS")]
+        [InlineData("GMT Standard Time", "Europe/London", "gb")]
+        [InlineData("GMT Standard Time", "Europe/Dublin", "ie")]
+        [InlineData("W. Europe Standard Time", "Europe/Rome", "it")]
+        [InlineData("New Zealand Standard Time", "Pacific/Auckland", "nz")]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/52072", TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.MacCatalyst)]
         public static void IdsConversionsWithRegionTest(string windowsId, string ianaId, string region)
         {