diff --git a/Sources/CoreFoundation/CFPlatform.c b/Sources/CoreFoundation/CFPlatform.c
index 8112cec632..1dee6cf195 100644
--- a/Sources/CoreFoundation/CFPlatform.c
+++ b/Sources/CoreFoundation/CFPlatform.c
@@ -1715,43 +1715,37 @@ typedef struct _CFThreadSpecificData {
 __stdcall
 #endif
 static void _CFThreadSpecificDestructor(void *ctx) {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_WIN32
     _CFThreadSpecificData *data = (_CFThreadSpecificData *)ctx;
     FlsSetValue(data->key, NULL);
     swift_release(data->value);
     free(data);
-#else
+#elif !TARGET_OS_WASI || defined(_REENTRANT)
     swift_release(ctx);
 #endif
-#endif
 }
 
 _CFThreadSpecificKey _CFThreadSpecificKeyCreate() {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
     _CFThreadSpecificKey key;
 #if TARGET_OS_WIN32
     key = FlsAlloc(_CFThreadSpecificDestructor);
-#else
+#elif !TARGET_OS_WASI || defined(_REENTRANT)
     pthread_key_create(&key, &_CFThreadSpecificDestructor);
-#endif
-    return key;
 #else
-    return 0;
+    key = 0;
 #endif
+    return key;
 }
 
 CFTypeRef _Nullable _CFThreadSpecificGet(_CFThreadSpecificKey key) {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_WIN32
     _CFThreadSpecificData *data = (_CFThreadSpecificData *)FlsGetValue(key);
     if (data == NULL) {
         return NULL;
     }
     return data->value;
-#else
+#elif !TARGET_OS_WASI || defined(_REENTRANT)
     return (CFTypeRef)pthread_getspecific(key);
-#endif
 #else
     return NULL;
 #endif
@@ -1761,7 +1755,6 @@ void _CFThreadSpecificSet(_CFThreadSpecificKey key, CFTypeRef _Nullable value) {
     // Intentionally not calling `swift_release` for previous value.
     // OperationQueue uses these API (through NSThreadSpecific), and balances
     // retain count manually.
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_WIN32
     free(FlsGetValue(key));
 
@@ -1778,7 +1771,7 @@ void _CFThreadSpecificSet(_CFThreadSpecificKey key, CFTypeRef _Nullable value) {
     }
 
     FlsSetValue(key, data);
-#else
+#elif !TARGET_OS_WASI || defined(_REENTRANT)
     if (value != NULL) {
         swift_retain((void *)value);
         pthread_setspecific(key, value);
@@ -1786,12 +1779,9 @@ void _CFThreadSpecificSet(_CFThreadSpecificKey key, CFTypeRef _Nullable value) {
         pthread_setspecific(key, NULL);
     }
 #endif
-#else
-#endif
 }
 
 _CFThreadRef _CFThreadCreate(const _CFThreadAttributes attrs, void *_Nullable (* _Nonnull startfn)(void *_Nullable), void *_CF_RESTRICT _Nullable context) {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_WIN32
     DWORD dwCreationFlags = 0;
     DWORD dwStackSize = 0;
@@ -1807,18 +1797,16 @@ _CFThreadRef _CFThreadCreate(const _CFThreadAttributes attrs, void *_Nullable (*
     return (_CFThreadRef)_beginthreadex(NULL, dwStackSize,
                                         (_beginthreadex_proc_type)startfn,
                                         context, dwCreationFlags, NULL);
-#else
+#elif !TARGET_OS_WASI || defined(_REENTRANT)
     _CFThreadRef thread;
     pthread_create(&thread, &attrs, startfn, context);
     return thread;
-#endif
 #else
     return NULL;
 #endif
 }
 
 CF_CROSS_PLATFORM_EXPORT int _CFThreadSetName(_CFThreadRef thread, const char *_Nonnull name) {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_MAC
     if (pthread_equal(pthread_self(), thread)) {
         return pthread_setname_np(name);
@@ -1848,15 +1836,14 @@ CF_CROSS_PLATFORM_EXPORT int _CFThreadSetName(_CFThreadRef thread, const char *_
 #elif TARGET_OS_BSD
     pthread_set_name_np(thread, name);
     return 0;
-#endif
-#else
+#elif TARGET_OS_WASI
+    // No thread naming support in WASI
     return -1;
 #endif
 }
 
 // `buf` must be null-terminated
 CF_CROSS_PLATFORM_EXPORT int _CFThreadGetName(char *buf, int length) {
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
 #if TARGET_OS_MAC
     return pthread_getname_np(pthread_self(), buf, length);
 #elif TARGET_OS_ANDROID
@@ -1902,11 +1889,11 @@ CF_CROSS_PLATFORM_EXPORT int _CFThreadGetName(char *buf, int length) {
     LocalFree(pszThreadDescription);
 
     return 0;
-#endif
-    return -1;
-#else
+#elif TARGET_OS_WASI
+    // No thread naming support in WASI
     return -1;
 #endif
+    return -1;
 }
 
 CF_EXPORT char **_CFEnviron(void) {
diff --git a/Sources/Foundation/NSLock.swift b/Sources/Foundation/NSLock.swift
index 938074d9e5..a154e03cc8 100644
--- a/Sources/Foundation/NSLock.swift
+++ b/Sources/Foundation/NSLock.swift
@@ -59,7 +59,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
 #endif
 
     public override init() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         InitializeSRWLock(mutex)
@@ -75,7 +75,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
     }
     
     deinit {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         // SRWLocks do not need to be explicitly destroyed
@@ -91,7 +91,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func lock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         AcquireSRWLockExclusive(mutex)
@@ -102,7 +102,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
 
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func unlock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         ReleaseSRWLockExclusive(mutex)
@@ -122,7 +122,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
 
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func `try`() -> Bool {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
         return true
 #elseif os(Windows)
@@ -134,7 +134,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func lock(before limit: Date) -> Bool {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         if TryAcquireSRWLockExclusive(mutex) != 0 {
@@ -146,7 +146,7 @@ open class NSLock: NSObject, NSLocking, @unchecked Sendable {
         }
 #endif
 
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
         return true
 #elseif os(macOS) || os(iOS) || os(Windows)
@@ -170,7 +170,7 @@ extension NSLock {
     }
 }
 
-#if SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if _runtime(_multithreaded)
 open class NSConditionLock : NSObject, NSLocking, @unchecked Sendable {
     internal var _cond = NSCondition()
     internal var _value: Int
@@ -282,7 +282,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
 
     public override init() {
         super.init()
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         InitializeCriticalSection(mutex)
@@ -312,7 +312,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
     }
     
     deinit {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         DeleteCriticalSection(mutex)
@@ -328,7 +328,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func lock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         EnterCriticalSection(mutex)
@@ -339,7 +339,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func unlock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         LeaveCriticalSection(mutex)
@@ -359,7 +359,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func `try`() -> Bool {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
         return true
 #elseif os(Windows)
@@ -371,7 +371,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func lock(before limit: Date) -> Bool {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         if TryEnterCriticalSection(mutex) {
@@ -383,7 +383,7 @@ open class NSRecursiveLock: NSObject, NSLocking, @unchecked Sendable {
         }
 #endif
 
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
         return true
 #elseif os(macOS) || os(iOS) || os(Windows)
@@ -404,7 +404,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     internal var cond = _ConditionVariablePointer.allocate(capacity: 1)
 
     public override init() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         InitializeSRWLock(mutex)
@@ -416,7 +416,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     }
     
     deinit {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         // SRWLock do not need to be explicitly destroyed
@@ -432,7 +432,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func lock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         AcquireSRWLockExclusive(mutex)
@@ -443,7 +443,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func unlock() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         ReleaseSRWLockExclusive(mutex)
@@ -454,7 +454,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func wait() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         SleepConditionVariableSRW(cond, mutex, WinSDK.INFINITE, 0)
@@ -465,7 +465,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
 
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func wait(until limit: Date) -> Bool {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
         return true
 #elseif os(Windows)
@@ -480,7 +480,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     
     @available(*, noasync, message: "Use async-safe scoped locking instead")
     open func signal() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         WakeConditionVariable(cond)
@@ -490,7 +490,7 @@ open class NSCondition: NSObject, NSLocking, @unchecked Sendable {
     }
     
     open func broadcast() {
-#if !SWIFT_CORELIBS_FOUNDATION_HAS_THREADS
+#if !_runtime(_multithreaded)
         // noop on no thread platforms
 #elseif os(Windows)
         WakeAllConditionVariable(cond)
diff --git a/Sources/Foundation/Thread.swift b/Sources/Foundation/Thread.swift
index ac1aa0eed4..a05534ac69 100644
--- a/Sources/Foundation/Thread.swift
+++ b/Sources/Foundation/Thread.swift
@@ -7,7 +7,6 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
-#if canImport(Dispatch)
 @_implementationOnly import CoreFoundation
 #if os(Windows)
 import WinSDK
@@ -19,6 +18,8 @@ import WinSDK
 @preconcurrency import Musl
 #elseif canImport(Bionic)
 @preconcurrency import Bionic
+#elseif canImport(WASILibc) && _runtime(_multithreaded)
+import wasi_pthread
 #endif
 
 // WORKAROUND_SR9811
@@ -124,7 +125,11 @@ open class Thread : NSObject {
     }
 
     open class func isMultiThreaded() -> Bool {
+#if _runtime(_multithreaded)
         return true
+#else
+        return false
+#endif
     }
 
     @available(*, noasync)
@@ -201,13 +206,18 @@ open class Thread : NSObject {
 #endif
     }
 
+    #if os(WASI)
+    @available(*, unavailable, message: "exit() is not available on WASI")
+    #endif
     @available(*, noasync)
     open class func exit() {
+#if !os(WASI)
         Thread.current._status = .finished
 #if os(Windows)
         ExitThread(0)
 #else
         pthread_exit(nil)
+#endif
 #endif
     }
 
@@ -248,7 +258,9 @@ open class Thread : NSObject {
 #if !os(Windows)
         let _ = withUnsafeMutablePointer(to: &_attr) { attr in
             pthread_attr_init(attr)
+            #if !os(WASI) // WASI does not support scheduling contention scope
             pthread_attr_setscope(attr, Int32(PTHREAD_SCOPE_SYSTEM))
+            #endif
             pthread_attr_setdetachstate(attr, Int32(PTHREAD_CREATE_DETACHED))
         }
 #endif
@@ -385,7 +397,7 @@ open class Thread : NSObject {
         let maxSupportedStackDepth = 128;
         let addrs = UnsafeMutablePointer<UnsafeMutableRawPointer?>.allocate(capacity: maxSupportedStackDepth)
         defer { addrs.deallocate() }
-#if os(Android) || os(OpenBSD) || canImport(Musl)
+#if os(Android) || os(OpenBSD) || canImport(Musl) || os(WASI)
         let count = 0
 #elseif os(Windows)
         let count = RtlCaptureStackBackTrace(0, DWORD(maxSupportedStackDepth),
@@ -408,7 +420,7 @@ open class Thread : NSObject {
     }
 
     open class var callStackSymbols: [String] {
-#if os(Android) || os(OpenBSD) || canImport(Musl)
+#if os(Android) || os(OpenBSD) || canImport(Musl) || os(WASI)
         return []
 #elseif os(Windows)
         let hProcess: HANDLE = GetCurrentProcess()
@@ -478,4 +490,3 @@ extension NSNotification.Name {
     public static let NSDidBecomeSingleThreaded = NSNotification.Name(rawValue: "NSDidBecomeSingleThreadedNotification")
     public static let NSThreadWillExit = NSNotification.Name(rawValue: "NSThreadWillExitNotification")
 }
-#endif