diff --git a/Sources/SwiftSyntaxMacrosTestSupport/Assertions.swift b/Sources/SwiftSyntaxMacrosTestSupport/Assertions.swift
index ffcb4d7e6a6..0b5125242b6 100644
--- a/Sources/SwiftSyntaxMacrosTestSupport/Assertions.swift
+++ b/Sources/SwiftSyntaxMacrosTestSupport/Assertions.swift
@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -64,6 +64,7 @@ public func assertMacroExpansion(
   file: StaticString = #filePath,
   line: UInt = #line
 ) {
+  _SwiftTestingDetection.runOnce(file: file, line: line)
   let specs = macros.mapValues { MacroSpec(type: $0) }
   assertMacroExpansion(
     originalSource,
@@ -113,6 +114,7 @@ public func assertMacroExpansion(
   file: StaticString = #filePath,
   line: UInt = #line
 ) {
+  _SwiftTestingDetection.runOnce(file: file, line: line)
   SwiftSyntaxMacrosGenericTestSupport.assertMacroExpansion(
     originalSource,
     expandedSource: expectedExpandedSource,
@@ -132,4 +134,4 @@ public func assertMacroExpansion(
     line: line,
     column: 0  // Not used in the failure handler
   )
-}
+}
\ No newline at end of file
diff --git a/Sources/SwiftSyntaxMacrosTestSupport/_SwiftTestingDetection.swift b/Sources/SwiftSyntaxMacrosTestSupport/_SwiftTestingDetection.swift
new file mode 100644
index 00000000000..d0942399e22
--- /dev/null
+++ b/Sources/SwiftSyntaxMacrosTestSupport/_SwiftTestingDetection.swift
@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+enum _SwiftTestingDetection {
+  private static let _lock = NSLock()
+  
+  // 'nonisolated(unsafe)' is required for Swift 6 strict concurrency checks 
+  // since we are manually synchronizing access via NSLock.
+  nonisolated(unsafe) private static var _executed = false
+  
+  static func runOnce(file: StaticString, line: UInt) {
+    _lock.lock()
+    defer { _lock.unlock() }
+    
+    // Ensure the detection logic and warning only run once per process.
+    if _executed { return }
+    _executed = true
+
+    let env = ProcessInfo.processInfo.environment
+
+    // Allow users to explicitly silence this warning via environment variable.
+    if env["SWIFTSYNTAX_TESTSUPPORT_SILENCE"] == "1" {
+      return
+    }
+    
+    // Safety check: If we are inside a legitimate XCTest environment, 
+    // do not show the warning.
+    let xctestEnvKeys = [
+      "XCTestConfigurationFilePath",
+      "XCTestBundlePath",
+      "XCTestSessionIdentifier",
+    ]
+    if xctestEnvKeys.contains(where: { env[$0] != nil }) {
+      return
+    }
+    
+    // Detect Swift Testing via environment variables or command line arguments.
+    let hasSwiftTestingEnv = env.keys.contains { $0.hasPrefix("SWIFT_TESTING_") }
+    let hasSwiftTestingArg = CommandLine.arguments.contains("--swift-testing")
+    let likelySwiftTesting = hasSwiftTestingEnv || hasSwiftTestingArg
+
+    guard likelySwiftTesting else {
+      return
+    }
+
+    // Strict mode: Treat the warning as a hard failure.
+    let strict = env["SWIFTSYNTAX_TESTSUPPORT_STRICT"] == "1"
+
+    let banner = """
+    \n━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+    SwiftSyntaxMacrosTestSupport is XCTest-based and appears to be invoked under Swift Testing.
+    This can cause silent failures and false positives (see #3190).
+    Until full Swift Testing support lands (see #2720), use XCTest or Testing-compatible helpers.
+    Controls: SWIFTSYNTAX_TESTSUPPORT_SILENCE=1 (silence), SWIFTSYNTAX_TESTSUPPORT_STRICT=1 (trap)
+    Location: \(file):\(line)
+    ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━\n
+    """
+
+    if strict {
+      preconditionFailure("SwiftSyntaxMacrosTestSupport used under Swift Testing. See #3190, #2720.")
+    } else {
+      if let data = banner.data(using: .utf8) {
+        // Use standard write(_:) for broad compatibility across macOS/Linux versions.
+        FileHandle.standardError.write(data)
+      }
+    }
+  }
+}
\ No newline at end of file