diff --git a/src/Tasks.UnitTests/GetReferencePaths_Tests.cs b/src/Tasks.UnitTests/GetReferencePaths_Tests.cs
index f4352c0322a..6e83c21d309 100644
--- a/src/Tasks.UnitTests/GetReferencePaths_Tests.cs
+++ b/src/Tasks.UnitTests/GetReferencePaths_Tests.cs
@@ -7,7 +7,9 @@
 using Microsoft.Build.Shared;
 using Microsoft.Build.Tasks;
 using Microsoft.Build.Utilities;
+using Shouldly;
 using Xunit;
+using Xunit.Abstractions;
 using FrameworkNameVersioning = System.Runtime.Versioning.FrameworkName;
 
 #if !RUNTIME_TYPE_NETCORE
@@ -20,6 +22,13 @@ namespace Microsoft.Build.UnitTests
     /// </summary>
     public sealed class GetReferenceAssmeblyPath_Tests
     {
+        private readonly ITestOutputHelper _output;
+
+        public GetReferenceAssmeblyPath_Tests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
         /// <summary>
         /// Test the case where there is a good target framework moniker passed in.
         /// </summary>
@@ -328,6 +337,91 @@ public void TestGeneralFrameworkMonikerGoodWithFrameworkInFallbackPaths()
                 Assert.Equal(".NET Framework 4.1", displayName);
             }
         }
+
+        /// <summary>
+        /// Test that a relative RootPath resolved via TaskEnvironment produces the same result as an absolute RootPath.
+        /// </summary>
+        [Fact]
+        public void TestRelativeRootPathProducesSameResultAsAbsolute()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            string baseDir = env.DefaultTestDirectory.Path;
+            string relativeDir = "framework-root";
+            string absoluteDir = Path.Combine(baseDir, relativeDir);
+            var framework41Directory = env.CreateFolder(Path.Combine(absoluteDir, Path.Combine("MyFramework", "v4.1") + Path.DirectorySeparatorChar));
+            var redistListDirectory = env.CreateFolder(Path.Combine(framework41Directory.Path, "RedistList"));
+            env.CreateFile(redistListDirectory, "FrameworkList.xml",
+                "<FileList Redist='Microsoft-Windows-CLRCoreComp' Name='.NET Framework 4.1'>" +
+                    "<File AssemblyName='System.Xml' Version='2.0.0.0' PublicKeyToken='b03f5f7f11d50a3a' Culture='Neutral' FileVersion='2.0.50727.208' InGAC='true' />" +
+                "</FileList >");
+
+            // Baseline: absolute RootPath
+            MockEngine absoluteEngine = new MockEngine();
+            GetReferenceAssemblyPaths absoluteTask = new GetReferenceAssemblyPaths();
+            absoluteTask.BuildEngine = absoluteEngine;
+            absoluteTask.TargetFrameworkMoniker = "MyFramework, Version=v4.1";
+            absoluteTask.RootPath = absoluteDir;
+            absoluteTask.Execute();
+
+            // Test: relative RootPath with TaskEnvironment
+            MockEngine relativeEngine = new MockEngine();
+            GetReferenceAssemblyPaths relativeTask = new GetReferenceAssemblyPaths();
+            relativeTask.BuildEngine = relativeEngine;
+            relativeTask.TargetFrameworkMoniker = "MyFramework, Version=v4.1";
+            relativeTask.TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment(baseDir);
+            relativeTask.RootPath = relativeDir;
+            relativeTask.Execute();
+
+            relativeTask.ReferenceAssemblyPaths.ShouldBe(absoluteTask.ReferenceAssemblyPaths);
+            relativeTask.TargetFrameworkMonikerDisplayName.ShouldBe(absoluteTask.TargetFrameworkMonikerDisplayName);
+            relativeEngine.Errors.ShouldBe(0);
+        }
+
+        /// <summary>
+        /// Test that a relative path in TargetFrameworkFallbackSearchPaths resolved via TaskEnvironment
+        /// produces the same result as an absolute fallback path.
+        /// </summary>
+        [Fact]
+        public void TestRelativeFallbackSearchPathProducesSameResultAsAbsolute()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            string baseDir = env.DefaultTestDirectory.Path;
+            string relativeDir = "framework-root";
+            string absoluteDir = Path.Combine(baseDir, relativeDir);
+            var framework41Directory = env.CreateFolder(Path.Combine(absoluteDir, Path.Combine("MyFramework", "v4.1") + Path.DirectorySeparatorChar));
+            var redistListDirectory = env.CreateFolder(Path.Combine(framework41Directory.Path, "RedistList"));
+            env.CreateFile(redistListDirectory, "FrameworkList.xml",
+                "<FileList Redist='Microsoft-Windows-CLRCoreComp' Name='.NET Framework 4.1'>" +
+                    "<File AssemblyName='System.Xml' Version='2.0.0.0' PublicKeyToken='b03f5f7f11d50a3a' Culture='Neutral' FileVersion='2.0.50727.208' InGAC='true' />" +
+                "</FileList >");
+
+            string nonExistentRoot = Path.Combine(baseDir, "nonexistent");
+
+            // Baseline: absolute fallback path
+            MockEngine absoluteEngine = new MockEngine();
+            GetReferenceAssemblyPaths absoluteTask = new GetReferenceAssemblyPaths();
+            absoluteTask.BuildEngine = absoluteEngine;
+            absoluteTask.TargetFrameworkMoniker = "MyFramework, Version=v4.1";
+            absoluteTask.RootPath = nonExistentRoot;
+            absoluteTask.TargetFrameworkFallbackSearchPaths = absoluteDir;
+            absoluteTask.Execute();
+
+            // Test: relative fallback path with TaskEnvironment
+            MockEngine relativeEngine = new MockEngine();
+            GetReferenceAssemblyPaths relativeTask = new GetReferenceAssemblyPaths();
+            relativeTask.BuildEngine = relativeEngine;
+            relativeTask.TargetFrameworkMoniker = "MyFramework, Version=v4.1";
+            relativeTask.TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment(baseDir);
+            relativeTask.RootPath = nonExistentRoot;
+            relativeTask.TargetFrameworkFallbackSearchPaths = relativeDir;
+            relativeTask.Execute();
+
+            relativeTask.ReferenceAssemblyPaths.ShouldBe(absoluteTask.ReferenceAssemblyPaths);
+            relativeTask.TargetFrameworkMonikerDisplayName.ShouldBe(absoluteTask.TargetFrameworkMonikerDisplayName);
+            relativeEngine.Errors.ShouldBe(0);
+        }
     }
 }
 #endif
diff --git a/src/Tasks/GetReferenceAssemblyPaths.cs b/src/Tasks/GetReferenceAssemblyPaths.cs
index 81b02cdf5a0..fd2cf698a3e 100644
--- a/src/Tasks/GetReferenceAssemblyPaths.cs
+++ b/src/Tasks/GetReferenceAssemblyPaths.cs
@@ -4,11 +4,12 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.Build.Framework;
+using Microsoft.Build.Shared;
 using Microsoft.Build.Utilities;
 using FrameworkNameVersioning = System.Runtime.Versioning.FrameworkName;
 
 #if FEATURE_GAC
-using Microsoft.Build.Shared;
+using System.Threading;
 using SystemProcessorArchitecture = System.Reflection.ProcessorArchitecture;
 #endif
 
@@ -19,8 +20,12 @@ namespace Microsoft.Build.Tasks
     /// <summary>
     /// Returns the reference assembly paths to the various frameworks
     /// </summary>
-    public class GetReferenceAssemblyPaths : TaskExtension
+    [MSBuildMultiThreadableTask]
+    public class GetReferenceAssemblyPaths : TaskExtension, IMultiThreadableTask
     {
+        /// <inheritdoc />
+        public TaskEnvironment TaskEnvironment { get; set; } = TaskEnvironment.Fallback;
+
         #region Data
 #if FEATURE_GAC
         /// <summary>
@@ -32,7 +37,23 @@ public class GetReferenceAssemblyPaths : TaskExtension
         /// <summary>
         /// Cache in a static whether or not we have found the 35sp1sentinel assembly.
         /// </summary>
-        private static bool? s_net35SP1SentinelAssemblyFound;
+        private static readonly Lazy<bool> s_net35SP1SentinelAssemblyFound = new Lazy<bool>(() =>
+        {
+            // get an assemblyname from the string representation of the sentinel assembly name
+            var sentinelAssemblyName = new AssemblyNameExtension(NET35SP1SentinelAssemblyName);
+            string path = GlobalAssemblyCache.GetLocation(
+                sentinelAssemblyName,
+                SystemProcessorArchitecture.MSIL,
+                runtimeVersion => "v2.0.50727",
+                new Version("2.0.57027"),
+                false,
+                new FileExists(p => FileUtilities.FileExistsNoThrow(p)),
+                GlobalAssemblyCache.pathFromFusionName,
+                GlobalAssemblyCache.gacEnumerator,
+                false);
+
+            return !string.IsNullOrEmpty(path);
+        }, LazyThreadSafetyMode.PublicationOnly);
 #endif
 
         /// <summary>
@@ -144,6 +165,11 @@ public string TargetFrameworkFallbackSearchPaths
         /// </summary>
         public override bool Execute()
         {
+            AbsolutePath? absoluteRootPath = !string.IsNullOrEmpty(RootPath)
+                ? TaskEnvironment.GetAbsolutePath(RootPath)
+                : new AbsolutePath(RootPath, ignoreRootedCheck: true);
+            IList<AbsolutePath> absoluteFallbackSearchPaths = ResolveAbsoluteFallbackSearchPaths(TargetFrameworkFallbackSearchPaths);
+
             FrameworkNameVersioning moniker;
             FrameworkNameVersioning monikerWithNoProfile = null;
 
@@ -169,16 +195,6 @@ public override bool Execute()
                 if (!BypassFrameworkInstallChecks && moniker.Identifier.Equals(".NETFramework", StringComparison.OrdinalIgnoreCase) &&
                     moniker.Version.Major < 4)
                 {
-                    // We have not got a value for whether or not the 35 sentinel assembly has been found
-                    if (!s_net35SP1SentinelAssemblyFound.HasValue)
-                    {
-                        // get an assemblyname from the string representation of the sentinel assembly name
-                        var sentinelAssemblyName = new AssemblyNameExtension(NET35SP1SentinelAssemblyName);
-
-                        string path = GlobalAssemblyCache.GetLocation(sentinelAssemblyName, SystemProcessorArchitecture.MSIL, runtimeVersion => "v2.0.50727", new Version("2.0.57027"), false, new FileExists(p => FileUtilities.FileExistsNoThrow(p)), GlobalAssemblyCache.pathFromFusionName, GlobalAssemblyCache.gacEnumerator, false);
-                        s_net35SP1SentinelAssemblyFound = !String.IsNullOrEmpty(path);
-                    }
-
                     // We did not find the SP1 sentinel assembly in the GAC. Therefore we must assume that SP1 isn't installed
                     if (!s_net35SP1SentinelAssemblyFound.Value)
                     {
@@ -195,7 +211,7 @@ public override bool Execute()
 
             try
             {
-                _tfmPaths = GetPaths(RootPath, TargetFrameworkFallbackSearchPaths, moniker);
+                _tfmPaths = GetPaths(absoluteRootPath, absoluteFallbackSearchPaths, moniker);
 
                 if (_tfmPaths?.Count > 0)
                 {
@@ -206,7 +222,7 @@ public override bool Execute()
                 // There is no point in generating the full framework paths if profile path could not be found.
                 if (targetingProfile && _tfmPaths != null)
                 {
-                    _tfmPathsNoProfile = GetPaths(RootPath, TargetFrameworkFallbackSearchPaths, monikerWithNoProfile);
+                    _tfmPathsNoProfile = GetPaths(absoluteRootPath, absoluteFallbackSearchPaths, monikerWithNoProfile);
                 }
 
                 // The path with out the profile is just the reference assembly paths.
@@ -236,14 +252,16 @@ public override bool Execute()
         /// <summary>
         /// Generate the set of chained reference assembly paths
         /// </summary>
-        private IList<String> GetPaths(string rootPath, string targetFrameworkFallbackSearchPaths, FrameworkNameVersioning frameworkmoniker)
+        private IList<String> GetPaths(AbsolutePath? rootPath, IList<AbsolutePath> fallbackSearchPaths, FrameworkNameVersioning frameworkmoniker)
         {
+            string fallbackSearchPathsJoined = string.Join(";", fallbackSearchPaths);
+
             IList<String> pathsToReturn = ToolLocationHelper.GetPathToReferenceAssemblies(
                                                 frameworkmoniker.Identifier,
                                                 frameworkmoniker.Version.ToString(),
                                                 frameworkmoniker.Profile,
-                                                rootPath,
-                                                targetFrameworkFallbackSearchPaths);
+                                                rootPath?.Value,
+                                                fallbackSearchPathsJoined);
 
             if (!SuppressNotFoundError)
             {
@@ -267,6 +285,26 @@ private IList<String> GetPaths(string rootPath, string targetFrameworkFallbackSe
             return pathsToReturn;
         }
 
+        /// <summary>
+        /// Resolves each semicolon-separated fallback search path to absolute via TaskEnvironment.
+        /// </summary>
+        private IList<AbsolutePath> ResolveAbsoluteFallbackSearchPaths(string fallbackSearchPaths)
+        {
+            if (string.IsNullOrEmpty(fallbackSearchPaths))
+            {
+                return [];
+            }
+
+            string[] parts = fallbackSearchPaths.Split(MSBuildConstants.SemicolonChar, StringSplitOptions.RemoveEmptyEntries);
+            var result = new AbsolutePath[parts.Length];
+            for (int i = 0; i < parts.Length; i++)
+            {
+                result[i] = TaskEnvironment.GetAbsolutePath(parts[i]);
+            }
+
+            return result;
+        }
+
         #endregion
     }
 }