diff --git a/zion/CMakeLists.txt b/zion/CMakeLists.txt
index fa2e22e..d1c5446 100644
--- a/zion/CMakeLists.txt
+++ b/zion/CMakeLists.txt
@@ -31,6 +31,7 @@ add_executable(zion
   object/port.cpp
   object/process.cpp
   object/reply_port.cpp
+  object/semaphore.cpp
   object/thread.cpp
   scheduler/context_switch.s
   scheduler/jump_user_space.s
diff --git a/zion/include/zcall.h b/zion/include/zcall.h
index a610454..7823650 100644
--- a/zion/include/zcall.h
+++ b/zion/include/zcall.h
@@ -64,5 +64,8 @@ SYS1(CapRelease, z_cap_t, cap);
 SYS1(MutexCreate, z_cap_t*, mutex_cap);
 SYS1(MutexLock, z_cap_t, mutex_cap);
 SYS1(MutexRelease, z_cap_t, mutex_cap);
+SYS1(SemaphoreCreate, z_cap_t*, semaphore_cap);
+SYS1(SemaphoreWait, z_cap_t, semaphore_cap);
+SYS1(SemaphoreSignal, z_cap_t, semaphore_cap);
 
 SYS1(Debug, const char*, message);
diff --git a/zion/include/ztypes.h b/zion/include/ztypes.h
index ebbb4c7..6dd9535 100644
--- a/zion/include/ztypes.h
+++ b/zion/include/ztypes.h
@@ -60,6 +60,9 @@ const uint64_t kZionCapRelease = 0x71;
 const uint64_t kZionMutexCreate = 0x80;
 const uint64_t kZionMutexLock = 0x81;
 const uint64_t kZionMutexRelease = 0x82;
+const uint64_t kZionSemaphoreCreate = 0x83;
+const uint64_t kZionSemaphoreWait = 0x84;
+const uint64_t kZionSemaphoreSignal = 0x85;
 
 // Debugging Calls.
 const uint64_t kZionDebug = 0x1'0000;
@@ -89,6 +92,9 @@ const uint64_t kZionPerm_SpawnThread = 0x200;
 // Permissions on mutexes.
 const uint64_t kZionPerm_Lock = 0x100;
 const uint64_t kZionPerm_Release = 0x200;
+// Permissions on semaphores.
+const uint64_t kZionPerm_Wait = 0x100;
+const uint64_t kZionPerm_Signal = 0x200;
 
 const z_perm_t kZionPerm_None = 0;
 const z_perm_t kZionPerm_All = -1;
diff --git a/zion/object/kernel_object.h b/zion/object/kernel_object.h
index 6aca277..2bdd477 100644
--- a/zion/object/kernel_object.h
+++ b/zion/object/kernel_object.h
@@ -15,6 +15,7 @@ class KernelObject : public glcr::RefCounted<KernelObject> {
     ENDPOINT = 0x7,
     REPLY_PORT = 0x8,
     MUTEX = 0x9,
+    SEMAPHORE = 0x10,
   };
 
   virtual uint64_t TypeTag() = 0;
diff --git a/zion/object/semaphore.cpp b/zion/object/semaphore.cpp
new file mode 100644
index 0000000..30f78cc
--- /dev/null
+++ b/zion/object/semaphore.cpp
@@ -0,0 +1,29 @@
+#include "object/semaphore.h"
+
+#include "scheduler/scheduler.h"
+
+glcr::RefPtr<Semaphore> Semaphore::Create() {
+  return glcr::AdoptPtr<Semaphore>(new Semaphore());
+}
+
+// FIXME: We almost certainly have some race conditions
+// between this and unlock where we could end up with
+// a thread in the blocked_threads_ queue while noone is holding the lock.
+void Semaphore::Signal() {
+  __atomic_fetch_add(&lock_, 0x1, __ATOMIC_SEQ_CST);
+  if (blocked_threads_.size() > 0) {
+    auto thread = blocked_threads_.PopFront();
+    thread->SetState(Thread::RUNNABLE);
+    gScheduler->Enqueue(thread);
+  }
+}
+
+void Semaphore::Wait() {
+  while (lock_ == 0) {
+    auto thread = gScheduler->CurrentThread();
+    thread->SetState(Thread::BLOCKED);
+    blocked_threads_.PushBack(thread);
+    gScheduler->Yield();
+  }
+  __atomic_fetch_sub(&lock_, 0x1, __ATOMIC_SEQ_CST);
+}
diff --git a/zion/object/semaphore.h b/zion/object/semaphore.h
new file mode 100644
index 0000000..8fd0bfe
--- /dev/null
+++ b/zion/object/semaphore.h
@@ -0,0 +1,33 @@
+#pragma once
+
+#include <glacier/container/intrusive_list.h>
+#include <glacier/memory/ref_ptr.h>
+
+#include "include/ztypes.h"
+#include "object/kernel_object.h"
+#include "object/thread.h"
+
+class Semaphore;
+
+template <>
+struct KernelObjectTag<Semaphore> {
+  static const uint64_t type = KernelObject::SEMAPHORE;
+};
+
+class Semaphore : public KernelObject {
+ public:
+  uint64_t TypeTag() override { return KernelObject::SEMAPHORE; }
+  static uint64_t DefaultPermissions() {
+    return kZionPerm_Wait | kZionPerm_Signal;
+  }
+
+  static glcr::RefPtr<Semaphore> Create();
+
+  void Wait();
+  void Signal();
+
+ private:
+  uint8_t lock_ = 0;
+
+  glcr::IntrusiveList<Thread> blocked_threads_;
+};
diff --git a/zion/syscall/synchronization.cpp b/zion/syscall/synchronization.cpp
index ed36879..c9f1174 100644
--- a/zion/syscall/synchronization.cpp
+++ b/zion/syscall/synchronization.cpp
@@ -1,6 +1,7 @@
 #include "syscall/synchronization.h"
 
 #include "object/mutex.h"
+#include "object/semaphore.h"
 #include "scheduler/scheduler.h"
 
 glcr::ErrorCode MutexCreate(ZMutexCreateReq* req) {
@@ -22,10 +23,35 @@ glcr::ErrorCode MutexLock(ZMutexLockReq* req) {
 glcr::ErrorCode MutexRelease(ZMutexReleaseReq* req) {
   auto& curr_proc = gScheduler->CurrentProcess();
   auto cap = curr_proc.GetCapability(req->mutex_cap);
-  // TODO: We may not want a separate permission for releasing the mutex.
   RET_ERR(ValidateCapability<Mutex>(cap, kZionPerm_Release));
 
   auto mutex = cap->obj<Mutex>();
   mutex->Release();
   return glcr::OK;
 }
+
+glcr::ErrorCode SemaphoreCreate(ZSemaphoreCreateReq* req) {
+  auto& curr_proc = gScheduler->CurrentProcess();
+  *req->semaphore_cap = curr_proc.AddNewCapability(Semaphore::Create());
+  return glcr::OK;
+}
+
+glcr::ErrorCode SemaphoreWait(ZSemaphoreWaitReq* req) {
+  auto& curr_proc = gScheduler->CurrentProcess();
+  auto cap = curr_proc.GetCapability(req->semaphore_cap);
+  RET_ERR(ValidateCapability<Semaphore>(cap, kZionPerm_Wait));
+
+  auto semaphore = cap->obj<Semaphore>();
+  semaphore->Wait();
+  return glcr::OK;
+}
+
+glcr::ErrorCode SemaphoreSignal(ZSemaphoreSignalReq* req) {
+  auto& curr_proc = gScheduler->CurrentProcess();
+  auto cap = curr_proc.GetCapability(req->semaphore_cap);
+  RET_ERR(ValidateCapability<Semaphore>(cap, kZionPerm_Signal));
+
+  auto semaphore = cap->obj<Semaphore>();
+  semaphore->Signal();
+  return glcr::OK;
+}
diff --git a/zion/syscall/synchronization.h b/zion/syscall/synchronization.h
index b508df2..4d1da24 100644
--- a/zion/syscall/synchronization.h
+++ b/zion/syscall/synchronization.h
@@ -7,3 +7,7 @@
 glcr::ErrorCode MutexCreate(ZMutexCreateReq* req);
 glcr::ErrorCode MutexLock(ZMutexLockReq* req);
 glcr::ErrorCode MutexRelease(ZMutexReleaseReq* req);
+
+glcr::ErrorCode SemaphoreCreate(ZSemaphoreCreateReq* req);
+glcr::ErrorCode SemaphoreWait(ZSemaphoreWaitReq* req);
+glcr::ErrorCode SemaphoreSignal(ZSemaphoreSignalReq* req);
diff --git a/zion/syscall/syscall.cpp b/zion/syscall/syscall.cpp
index 89ee5b8..52b0f81 100644
--- a/zion/syscall/syscall.cpp
+++ b/zion/syscall/syscall.cpp
@@ -88,6 +88,9 @@ extern "C" z_err_t SyscallHandler(uint64_t call_id, void* req) {
     CASE(MutexCreate);
     CASE(MutexLock);
     CASE(MutexRelease);
+    CASE(SemaphoreCreate);
+    CASE(SemaphoreWait);
+    CASE(SemaphoreSignal);
     // syscall/debug.h
     CASE(Debug);
     default: