diff --git a/pkg/analyzer/lib/src/workspace/pub.dart b/pkg/analyzer/lib/src/workspace/pub.dart
index 0daefaeb873..841281ebc90 100644
--- a/pkg/analyzer/lib/src/workspace/pub.dart
+++ b/pkg/analyzer/lib/src/workspace/pub.dart
@@ -68,6 +68,9 @@ class PubWorkspace extends SimpleWorkspace {
     for (var current in start.withAncestors) {
       var pubspec = current.getChildAssumingFile(file_paths.pubspecYaml);
       if (pubspec.exists) {
+        if (_isInThirdPartyDart(pubspec)) {
+          return null;
+        }
         var root = current.path;
         return PubWorkspace._(provider, packages, root, pubspec);
       }
@@ -83,6 +86,19 @@ class PubWorkspace extends SimpleWorkspace {
       return null;
     }
   }
+
+  /// See https://buganizer.corp.google.com/issues/273584249
+  ///
+  /// Check if `/home/workspace/third_party/dart/my/pubspec.yaml`
+  /// If so, we are in a Blaze workspace, and should not create Pub.
+  static bool _isInThirdPartyDart(File pubspec) {
+    final path = pubspec.path;
+    final pathContext = pubspec.provider.pathContext;
+    final pathComponents = pathContext.split(path);
+    return pathComponents.length > 4 &&
+        pathComponents[pathComponents.length - 3] == 'dart' &&
+        pathComponents[pathComponents.length - 4] == 'third_party';
+  }
 }
 
 /// Information about a package defined in a [PubWorkspace].
diff --git a/pkg/analyzer/test/src/dart/analysis/context_locator_test.dart b/pkg/analyzer/test/src/dart/analysis/context_locator_test.dart
index 0f36b26c4ef..11e0b10d05a 100644
--- a/pkg/analyzer/test/src/dart/analysis/context_locator_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/context_locator_test.dart
@@ -1636,6 +1636,35 @@ ${getFolder(outPath).path}
     expect(contentRoot.packagesFile, packageConfigJsonFile);
   }
 
+  /// See https://buganizer.corp.google.com/issues/273584249
+  void test_locateRoots_single_directory_blaze_hasPubspecYaml_thirdPartyDart() {
+    final workspacePath = '/home/workspace';
+    final thirdPartyDartPath = '$workspacePath/third_party/dart';
+
+    final myPackagePath = '$thirdPartyDartPath/my';
+    final myPackage = getFolder(myPackagePath);
+
+    newFile('$workspacePath/${file_paths.blazeWorkspaceMarker}', '');
+    final buildFile = newBlazeBuildFile(myPackagePath, '');
+    final pubspecYamlFile = newPubspecYamlFile(myPackagePath, '');
+    final myFile = newFile('$myPackagePath/lib/my.dart', '');
+
+    final roots = contextLocator.locateRoots(
+      includedPaths: [
+        myPackage.path,
+      ],
+    );
+    expect(roots, hasLength(1));
+
+    final root = findRoot(roots, myPackage);
+    expect(root.includedPaths, unorderedEquals([myPackage.path]));
+    expect(root.excludedPaths, isEmpty);
+    expect(root.optionsFile, isNull);
+    expect(root.packagesFile, isNull);
+    _assertBlazeWorkspace(root.workspace, workspacePath);
+    _assertAnalyzedFiles2(root, [buildFile, pubspecYamlFile, myFile]);
+  }
+
   void test_locateRoots_single_file_gnWorkspace() {
     var workspaceRootPath = '/workspace';
     newFolder(workspaceRootPath);