diff --git a/eng/testing/tests.browser.targets b/eng/testing/tests.browser.targets
index bce044984e9379..982b8589e76c14 100644
--- a/eng/testing/tests.browser.targets
+++ b/eng/testing/tests.browser.targets
@@ -94,6 +94,7 @@
     <WasmXHarnessMonoArgs Condition="'$(XunitShowProgress)' == 'true'">$(WasmXHarnessMonoArgs) --setenv=XHARNESS_LOG_TEST_START=true</WasmXHarnessMonoArgs>
     <!-- help unit test with PlatformDetection.IsThreadingSupported via IsBrowserThreadingSupported env variable -->
     <WasmXHarnessMonoArgs Condition="'$(WasmEnableThreads)' == 'true'">$(WasmXHarnessMonoArgs) --setenv=IsBrowserThreadingSupported=true</WasmXHarnessMonoArgs>
+    <WasmXHarnessMaxParallelThreads Condition="'$(WasmEnableThreads)' == 'true' and '$(WasmXHarnessMaxParallelThreads)' == ''">8</WasmXHarnessMaxParallelThreads>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(RunScriptCommand)' == ''">
@@ -112,6 +113,7 @@
     <_XHarnessArgs Condition="'$(WasmXHarnessArgsCli)' != ''"      >$(_XHarnessArgs) $(WasmXHarnessArgsCli)</_XHarnessArgs>
 
     <_AppArgs Condition="'$(WasmEnableThreads)' == 'true'">$(_AppArgs) -threads</_AppArgs>
+    <_AppArgs Condition="'$(WasmXHarnessMaxParallelThreads)' != ''">$(_AppArgs) -parallelThreads $(WasmXHarnessMaxParallelThreads)</_AppArgs>
     <!-- There are two flavors of WasmXHarnessArgs and WasmXHarnessMonoArgs, one is MSBuild property and the other is environment variable -->
     <RunScriptCommand Condition="'$(OS)' != 'Windows_NT'">$HARNESS_RUNNER $(_XHarnessArgs) %24XHARNESS_ARGS %24WasmXHarnessArgs -- $(WasmXHarnessMonoArgs) %24WasmXHarnessMonoArgs $(_AppArgs) %24WasmTestAppArgs</RunScriptCommand>
     <RunScriptCommand Condition="'$(OS)' == 'Windows_NT'">%HARNESS_RUNNER% $(_XHarnessArgs) %XHARNESS_ARGS% %WasmXHarnessArgs%  -- $(WasmXHarnessMonoArgs) %WasmXHarnessMonoArgs% $(_AppArgs) %WasmTestAppArgs%</RunScriptCommand>
diff --git a/src/libraries/Common/tests/WasmTestRunner/WasmTestRunner.cs b/src/libraries/Common/tests/WasmTestRunner/WasmTestRunner.cs
index 2eb1c1e440c33b..9935182c69d038 100644
--- a/src/libraries/Common/tests/WasmTestRunner/WasmTestRunner.cs
+++ b/src/libraries/Common/tests/WasmTestRunner/WasmTestRunner.cs
@@ -9,8 +9,8 @@
 
 public class WasmTestRunner : WasmApplicationEntryPoint
 {
-    // TODO: Set max threads for run in parallel
-    // protected override int? MaxParallelThreads => RunInParallel ? 8 : base.MaxParallelThreads;
+    protected int MaxParallelThreadsFromArg { get; set; }
+    protected override int? MaxParallelThreads => RunInParallel ? MaxParallelThreadsFromArg : base.MaxParallelThreads;
 
     public static async Task<int> Main(string[] args)
     {
@@ -65,9 +65,11 @@ public static async Task<int> Main(string[] args)
                     break;
                 case "-threads":
                     runner.IsThreadless = false;
-                    // TODO: Enable run in parallel
-                    // runner.RunInParallel = true;
-                    // Console.WriteLine($"Running in parallel with {runner.MaxParallelThreads} threads.");
+                    break;
+                case "-parallelThreads":
+                    runner.MaxParallelThreadsFromArg = Math.Max(1, int.Parse(args[i + 1]));
+                    runner.RunInParallel = runner.MaxParallelThreadsFromArg > 1;
+                    i++;
                     break;
                 case "-verbosity":
                     runner.MinimumLogLevel = Enum.Parse<MinimumLogLevel>(args[i + 1]);
@@ -105,4 +107,12 @@ public static async Task<int> Main(string[] args)
 
         return res;
     }
+
+    public override Task RunAsync()
+    {
+        if (RunInParallel)
+            Console.WriteLine($"Running in parallel with {MaxParallelThreads} threads.");
+
+        return base.RunAsync();
+    }
 }
diff --git a/src/libraries/System.Net.WebSockets.Client/tests/System.Net.WebSockets.Client.Tests.csproj b/src/libraries/System.Net.WebSockets.Client/tests/System.Net.WebSockets.Client.Tests.csproj
index 0292fd4c043532..2c7762cc6bc861 100644
--- a/src/libraries/System.Net.WebSockets.Client/tests/System.Net.WebSockets.Client.Tests.csproj
+++ b/src/libraries/System.Net.WebSockets.Client/tests/System.Net.WebSockets.Client.Tests.csproj
@@ -17,6 +17,7 @@
 
     <!-- This WASM test is problematic and slow right now. This sets the xharness timeout but there is also override in sendtohelix-browser.targets -->
     <WasmXHarnessTestsTimeout>01:15:00</WasmXHarnessTestsTimeout>
+    <WasmXHarnessMaxParallelThreads>1</WasmXHarnessMaxParallelThreads>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System.Runtime.InteropServices.JavaScript.Tests.csproj b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System.Runtime.InteropServices.JavaScript.Tests.csproj
index e21c5d8c5e16dc..135b1b78297ab8 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System.Runtime.InteropServices.JavaScript.Tests.csproj
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System.Runtime.InteropServices.JavaScript.Tests.csproj
@@ -14,6 +14,7 @@
     <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
     <!-- to see timing and which test aborted the runtime -->
     <XunitShowProgress>true</XunitShowProgress>
+    <WasmXHarnessMaxParallelThreads>1</WasmXHarnessMaxParallelThreads>
   </PropertyGroup>
   <!-- Make debugging easier -->
   <PropertyGroup Condition="'$(Configuration)' == 'Debug'">
diff --git a/src/libraries/System.Runtime/tests/System.Buffers.Tests/System.Buffers.Tests.csproj b/src/libraries/System.Runtime/tests/System.Buffers.Tests/System.Buffers.Tests.csproj
index 10b79281bf52b6..66ae6833a50eb5 100644
--- a/src/libraries/System.Runtime/tests/System.Buffers.Tests/System.Buffers.Tests.csproj
+++ b/src/libraries/System.Runtime/tests/System.Buffers.Tests/System.Buffers.Tests.csproj
@@ -5,6 +5,9 @@
     <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
     <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
   </PropertyGroup>
+  <PropertyGroup Condition="'$(TargetOS)' == 'browser'">
+    <WasmXHarnessMaxParallelThreads>1</WasmXHarnessMaxParallelThreads>
+  </PropertyGroup>
   <ItemGroup>
     <Compile Include="ArrayPool\ArrayPoolTest.cs" />
     <Compile Include="ArrayPool\CollectionTests.cs" />
diff --git a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj
index ddb87ab85f5745..61bea23cac8200 100644
--- a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj
+++ b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/System.Text.Json.Tests.csproj
@@ -21,6 +21,7 @@
     <XunitShowProgress>true</XunitShowProgress>
     <!-- This WASM test is problematic and slow right now. This sets the xharness timeout but there is also override in sendtohelix-browser.targets -->
     <WasmXHarnessTestsTimeout>01:15:00</WasmXHarnessTestsTimeout>
+    <WasmXHarnessMaxParallelThreads>1</WasmXHarnessMaxParallelThreads>
   </PropertyGroup>
   <ItemGroup Condition="'$(ContinuousIntegrationBuild)' == 'true'">
     <HighAotMemoryUsageAssembly Include="System.Text.Json.Tests.dll" />