diff --git a/Package.swift b/Package.swift
index 40e28c845..52aa87831 100644
--- a/Package.swift
+++ b/Package.swift
@@ -123,6 +123,8 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .enableUpcomingFeature("InternalImportsByDefault"),
 
       .define("SWT_TARGET_OS_APPLE", .when(platforms: [.macOS, .iOS, .macCatalyst, .watchOS, .tvOS, .visionOS])),
+
+      .define("SWT_NO_FILE_IO", .when(platforms: [.wasi])),
     ]
   }
 
diff --git a/Package@swift-6.0.swift b/Package@swift-6.0.swift
index 71b1fd1e7..a2006b1e6 100644
--- a/Package@swift-6.0.swift
+++ b/Package@swift-6.0.swift
@@ -123,6 +123,8 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .enableUpcomingFeature("InternalImportsByDefault"),
 
       .define("SWT_TARGET_OS_APPLE", .when(platforms: [.macOS, .iOS, .macCatalyst, .watchOS, .tvOS, .visionOS])),
+
+      .define("SWT_NO_FILE_IO", .when(platforms: [.wasi])),
     ]
   }
 
diff --git a/Sources/Testing/Events/Recorder/Event.Symbol.swift b/Sources/Testing/Events/Recorder/Event.Symbol.swift
index 719c3e795..10f0fbf93 100644
--- a/Sources/Testing/Events/Recorder/Event.Symbol.swift
+++ b/Sources/Testing/Events/Recorder/Event.Symbol.swift
@@ -100,7 +100,7 @@ extension Event.Symbol {
   /// be used to represent it in text-based output. The value of this property
   /// is platform-dependent.
   public var unicodeCharacter: Character {
-#if SWT_TARGET_OS_APPLE || os(Linux)
+#if SWT_TARGET_OS_APPLE || os(Linux) || os(WASI)
     switch self {
     case .default:
       // Unicode: WHITE DIAMOND
diff --git a/Sources/Testing/Events/TimeValue.swift b/Sources/Testing/Events/TimeValue.swift
index f798a78ef..033aa9f86 100644
--- a/Sources/Testing/Events/TimeValue.swift
+++ b/Sources/Testing/Events/TimeValue.swift
@@ -75,6 +75,10 @@ extension TimeValue: Codable {}
 
 extension TimeValue: CustomStringConvertible {
   var description: String {
+#if os(WASI)
+    // BUG: https://github.com/apple/swift/issues/72398
+    return String(describing: Duration(self))
+#else
     let (secondsFromAttoseconds, attosecondsRemaining) = attoseconds.quotientAndRemainder(dividingBy: 1_000_000_000_000_000_000)
     let seconds = seconds + secondsFromAttoseconds
     var milliseconds = attosecondsRemaining / 1_000_000_000_000_000
@@ -88,6 +92,7 @@ extension TimeValue: CustomStringConvertible {
       }
       return String(cString: buffer.baseAddress!)
     }
+#endif
   }
 }
 
diff --git a/Sources/Testing/SourceAttribution/Backtrace.swift b/Sources/Testing/SourceAttribution/Backtrace.swift
index 3d9977c76..df8834477 100644
--- a/Sources/Testing/SourceAttribution/Backtrace.swift
+++ b/Sources/Testing/SourceAttribution/Backtrace.swift
@@ -72,6 +72,10 @@ public struct Backtrace: Sendable {
         initializedCount = .init(backtrace(addresses.baseAddress!, .init(addresses.count)))
 #elseif os(Windows)
         initializedCount = Int(RtlCaptureStackBackTrace(0, ULONG(addresses.count), addresses.baseAddress!, nil))
+#elseif os(WASI)
+        // SEE: https://github.com/WebAssembly/WASI/issues/159
+        // SEE: https://github.com/apple/swift/pull/31693
+        initializedCount = 0
 #else
 #warning("Platform-specific implementation missing: backtraces unavailable")
         initializedCount = 0
diff --git a/Sources/Testing/Support/Environment.swift b/Sources/Testing/Support/Environment.swift
index fb01d95bc..d9fa050b3 100644
--- a/Sources/Testing/Support/Environment.swift
+++ b/Sources/Testing/Support/Environment.swift
@@ -36,7 +36,7 @@ enum Environment {
   static func variable(named name: String) -> String? {
 #if SWT_NO_ENVIRONMENT_VARIABLES
     simulatedEnvironment.rawValue[name]
-#elseif SWT_TARGET_OS_APPLE || os(Linux)
+#elseif SWT_TARGET_OS_APPLE || os(Linux) || os(WASI)
     getenv(name).flatMap { String(validatingUTF8: $0) }
 #elseif os(Windows)
     name.withCString(encodedAs: UTF16.self) { name in
diff --git a/Sources/Testing/Support/FileHandle.swift b/Sources/Testing/Support/FileHandle.swift
index 247ada218..adab81981 100644
--- a/Sources/Testing/Support/FileHandle.swift
+++ b/Sources/Testing/Support/FileHandle.swift
@@ -140,7 +140,7 @@ struct FileHandle: ~Copyable, Sendable {
       let fd: CInt = -1
 #endif
 
-      if fd >= 0 {
+      if Bool(fd >= 0) {
         return try body(fd)
       }
       return try body(nil)
diff --git a/Sources/Testing/Support/Locked.swift b/Sources/Testing/Support/Locked.swift
index 5c9fddde7..05bef558a 100644
--- a/Sources/Testing/Support/Locked.swift
+++ b/Sources/Testing/Support/Locked.swift
@@ -40,6 +40,8 @@ struct Locked<T>: RawRepresentable, Sendable where T: Sendable {
   private typealias _Lock = pthread_mutex_t
 #elseif os(Windows)
   private typealias _Lock = SRWLOCK
+#elseif os(WASI)
+  // No locks on WASI.
 #else
 #warning("Platform-specific implementation missing: locking unavailable")
   private typealias _Lock = Void
@@ -53,6 +55,8 @@ struct Locked<T>: RawRepresentable, Sendable where T: Sendable {
         _ = pthread_mutex_destroy(lock)
 #elseif os(Windows)
         // No deinitialization needed.
+#elseif os(WASI)
+        // No locks on WASI.
 #else
 #warning("Platform-specific implementation missing: locking unavailable")
 #endif
@@ -70,6 +74,8 @@ struct Locked<T>: RawRepresentable, Sendable where T: Sendable {
       _ = pthread_mutex_init(lock, nil)
 #elseif os(Windows)
       InitializeSRWLock(lock)
+#elseif os(WASI)
+      // No locks on WASI.
 #else
 #warning("Platform-specific implementation missing: locking unavailable")
 #endif
@@ -105,6 +111,8 @@ struct Locked<T>: RawRepresentable, Sendable where T: Sendable {
       defer {
         ReleaseSRWLockExclusive(lock)
       }
+#elseif os(WASI)
+      // No locks on WASI.
 #else
 #warning("Platform-specific implementation missing: locking unavailable")
 #endif
diff --git a/Sources/Testing/Support/Versions.swift b/Sources/Testing/Support/Versions.swift
index 8c4251bc4..5fe79f02e 100644
--- a/Sources/Testing/Support/Versions.swift
+++ b/Sources/Testing/Support/Versions.swift
@@ -31,7 +31,7 @@ let operatingSystemVersion: String = {
   default:
     return "\(productVersion) (\(buildNumber))"
   }
-#elseif !SWT_NO_UNAME && (SWT_TARGET_OS_APPLE || os(Linux))
+#elseif !SWT_NO_UNAME && (SWT_TARGET_OS_APPLE || os(Linux) || os(WASI))
   var name = utsname()
   if 0 == uname(&name) {
     let release = withUnsafeBytes(of: name.release) { release in
diff --git a/Sources/TestingInternals/Discovery.cpp b/Sources/TestingInternals/Discovery.cpp
index bba05ffdc..60d482993 100644
--- a/Sources/TestingInternals/Discovery.cpp
+++ b/Sources/TestingInternals/Discovery.cpp
@@ -260,7 +260,7 @@ static void enumerateTypeMetadataSections(const SectionEnumerator& body) {
   }
 }
 
-#elif defined(__linux__) || defined(_WIN32)
+#elif defined(__linux__) || defined(_WIN32) || defined(__wasi__)
 #pragma mark - Linux/Windows implementation
 
 /// Specifies the address range corresponding to a section.
diff --git a/Tests/TestingTests/Support/EnvironmentTests.swift b/Tests/TestingTests/Support/EnvironmentTests.swift
index b25d4528c..66e1c838a 100644
--- a/Tests/TestingTests/Support/EnvironmentTests.swift
+++ b/Tests/TestingTests/Support/EnvironmentTests.swift
@@ -79,7 +79,7 @@ extension Environment {
       environment[name] = value
     }
     return true
-#elseif SWT_TARGET_OS_APPLE || os(Linux)
+#elseif SWT_TARGET_OS_APPLE || os(Linux) || os(WASI)
     if let value {
       return 0 == setenv(name, value, 1)
     }