diff --git a/lib/concurrent/atomic/semaphore.rb b/lib/concurrent/atomic/semaphore.rb
new file mode 100644
index 000000000..db907d3f1
--- /dev/null
+++ b/lib/concurrent/atomic/semaphore.rb
@@ -0,0 +1,226 @@
+require 'concurrent/atomic/condition'
+
+module Concurrent
+  class MutexSemaphore
+    # @!macro [attach] semaphore_method_initialize
+    #
+    #   Create a new `Semaphore` with the initial `count`.
+    #
+    #   @param [Fixnum] count the initial count
+    #
+    #   @raise [ArgumentError] if `count` is not an integer or is less than zero
+    def initialize(count)
+      unless count.is_a?(Fixnum) && count >= 0
+        fail ArgumentError, 'count must be an non-negative integer'
+      end
+      @mutex = Mutex.new
+      @condition = Condition.new
+      @free = count
+    end
+
+    # @!macro [attach] semaphore_method_acquire
+    #
+    #   Acquires the given number of permits from this semaphore,
+    #     blocking until all are available.
+    #
+    #   @param [Fixnum] permits Number of permits to acquire
+    #
+    #   @raise [ArgumentError] if `permits` is not an integer or is less than
+    #     one
+    #
+    #   @return [Nil]
+    def acquire(permits = 1)
+      unless permits.is_a?(Fixnum) && permits > 0
+        fail ArgumentError, 'permits must be an integer greater than zero'
+      end
+      @mutex.synchronize do
+        try_acquire_timed(permits, nil)
+        nil
+      end
+    end
+
+    # @!macro [attach] semaphore_method_available_permits
+    #
+    #   Returns the current number of permits available in this semaphore.
+    #
+    #   @return [Integer]
+    def available_permits
+      @mutex.synchronize { @free }
+    end
+
+    # @!macro [attach] semaphore_method_drain_permits
+    #
+    #   Acquires and returns all permits that are immediately available.
+    #
+    #   @return [Integer]
+    def drain_permits
+      @mutex.synchronize do
+        @free.tap { |_| @free = 0 }
+      end
+    end
+
+    # @!macro [attach] semaphore_method_try_acquire
+    #
+    #   Acquires the given number of permits from this semaphore,
+    #     only if all are available at the time of invocation or within
+    #     `timeout` interval
+    #
+    #   @param [Fixnum] permits the number of permits to acquire
+    #
+    #   @param [Fixnum] timeout the number of seconds to wait for the counter
+    #     or `nil` to return immediately
+    #
+    #   @raise [ArgumentError] if `permits` is not an integer or is less than
+    #     one
+    #
+    #   @return [Boolean] `false` if no permits are available, `true` when
+    #     acquired a permit
+    def try_acquire(permits = 1, timeout = nil)
+      unless permits.is_a?(Fixnum) && permits > 0
+        fail ArgumentError, 'permits must be an integer greater than zero'
+      end
+      @mutex.synchronize do
+        if timeout.nil?
+          try_acquire_now(permits)
+        else
+          try_acquire_timed(permits, timeout)
+        end
+      end
+    end
+
+    # @!macro [attach] semaphore_method_release
+    #
+    #   Releases the given number of permits, returning them to the semaphore.
+    #
+    #   @param [Fixnum] permits Number of permits to return to the semaphore.
+    #
+    #   @raise [ArgumentError] if `permits` is not a number or is less than one
+    #
+    #   @return [Nil]
+    def release(permits = 1)
+      unless permits.is_a?(Fixnum) && permits > 0
+        fail ArgumentError, 'permits must be an integer greater than zero'
+      end
+      @mutex.synchronize do
+        @free += permits
+        permits.times { @condition.signal }
+      end
+      nil
+    end
+
+    # @!macro [attach] semaphore_method_reduce_permits
+    # 
+    #   @api private
+    #
+    #   Shrinks the number of available permits by the indicated reduction.
+    #
+    #   @param [Fixnum] reduction Number of permits to remove.
+    #
+    #   @raise [ArgumentError] if `reduction` is not an integer or is negative
+    #
+    #   @raise [ArgumentError] if `@free` - `@reduction` is less than zero
+    #
+    #   @return [Nil]
+    def reduce_permits(reduction)
+      unless reduction.is_a?(Fixnum) && reduction >= 0
+        fail ArgumentError, 'reduction must be an non-negative integer'
+      end
+      @mutex.synchronize { @free -= reduction }
+      nil 
+    end
+
+    private
+
+    def try_acquire_now(permits)
+      if @free >= permits
+        @free -= permits
+        true
+      else
+        false
+      end
+    end
+
+    def try_acquire_timed(permits, timeout)
+      remaining = Condition::Result.new(timeout)
+      while !try_acquire_now(permits) && remaining.can_wait?
+        @condition.signal
+        remaining = @condition.wait(@mutex, remaining.remaining_time)
+      end
+      remaining.can_wait? ? true : false
+    end
+  end
+
+  if RUBY_PLATFORM == 'java'
+
+    # @!macro semaphore
+    class JavaSemaphore
+      # @!macro semaphore_method_initialize
+      def initialize(count)
+        unless count.is_a?(Fixnum) && count >= 0
+          fail(ArgumentError,
+               'count must be in integer greater than or equal zero')
+        end
+        @semaphore = java.util.concurrent.Semaphore.new(count)
+      end
+
+      # @!macro semaphore_method_acquire
+      def acquire(permits = 1)
+        unless permits.is_a?(Fixnum) && permits > 0
+          fail ArgumentError, 'permits must be an integer greater than zero'
+        end
+        @semaphore.acquire(permits)
+      end
+
+      # @!macro semaphore_method_available_permits
+      def available_permits
+        @semaphore.availablePermits
+      end
+
+      # @!macro semaphore_method_drain_permits
+      def drain_permits
+        @semaphore.drainPermits
+      end
+
+      # @!macro semaphore_method_try_acquire
+      def try_acquire(permits = 1, timeout = nil)
+        unless permits.is_a?(Fixnum) && permits > 0
+          fail ArgumentError, 'permits must be an integer greater than zero'
+        end
+        if timeout.nil?
+          @semaphore.tryAcquire(permits)
+        else
+          @semaphore.tryAcquire(permits,
+                                 timeout,
+                                 java.util.concurrent.TimeUnit::SECONDS)
+        end
+      end
+
+      # @!macro semaphore_method_release
+      def release(permits = 1)
+        unless permits.is_a?(Fixnum) && permits > 0
+          fail ArgumentError, 'permits must be an integer greater than zero'
+        end
+        @semaphore.release(permits)
+        true
+      end
+
+      # @!macro semaphore_method_reduce_permits
+      def reduce_permits(reduction)
+        unless reduction.is_a?(Fixnum) && reduction >= 0
+          fail ArgumentError, 'reduction must be an non-negative integer'
+        end
+        @semaphore.reducePermits(reduction)
+      end
+    end
+
+    # @!macro semaphore
+    class Semaphore < JavaSemaphore
+    end
+
+  else
+
+    # @!macro semaphore
+    class Semaphore < MutexSemaphore
+    end
+  end
+end
diff --git a/lib/concurrent/atomics.rb b/lib/concurrent/atomics.rb
index 159d2fbea..a54684644 100644
--- a/lib/concurrent/atomics.rb
+++ b/lib/concurrent/atomics.rb
@@ -8,3 +8,4 @@
 require 'concurrent/atomic/count_down_latch'
 require 'concurrent/atomic/event'
 require 'concurrent/atomic/synchronization'
+require 'concurrent/atomic/semaphore'
diff --git a/spec/concurrent/atomic/semaphore_spec.rb b/spec/concurrent/atomic/semaphore_spec.rb
new file mode 100755
index 000000000..e2eba8743
--- /dev/null
+++ b/spec/concurrent/atomic/semaphore_spec.rb
@@ -0,0 +1,166 @@
+require 'spec_helper'
+
+shared_examples :semaphore do
+  let(:semaphore) { described_class.new(3) }
+
+  context '#initialize' do
+    it 'raises an exception if the initial count is not an integer' do
+      expect {
+        described_class.new('foo')
+      }.to raise_error(ArgumentError)
+    end
+  end
+
+  describe '#acquire' do
+    context 'permits available' do
+      it 'should return true immediately' do
+        result = semaphore.acquire
+        expect(result).to be_nil
+      end
+    end
+
+    context 'not enough permits available' do
+      it 'should block thread until permits are available' do
+        semaphore.drain_permits
+        Thread.new { sleep(0.2) && semaphore.release }
+
+        result = semaphore.acquire
+        expect(result).to be_nil
+        expect(semaphore.available_permits).to eq 0
+      end
+    end
+  end
+
+  describe '#drain_permits' do
+    it 'drains all available permits' do
+      drained = semaphore.drain_permits
+      expect(drained).to eq 3
+      expect(semaphore.available_permits).to eq 0
+    end
+
+    it 'drains nothing in no permits are available' do
+      semaphore.reduce_permits 3
+      drained = semaphore.drain_permits
+      expect(drained).to eq 0
+    end
+  end
+
+  describe '#try_acquire' do
+    context 'without timeout' do
+      it 'acquires immediately if permits are available' do
+        result = semaphore.try_acquire(1)
+        expect(result).to be_truthy
+      end
+
+      it 'returns false immediately in no permits are available' do
+        result = semaphore.try_acquire(20)
+        expect(result).to be_falsey
+      end
+    end
+
+    context 'with timeout' do
+      it 'acquires immediately if permits are available' do
+        result = semaphore.try_acquire(1, 5)
+        expect(result).to be_truthy
+      end
+
+      it 'acquires when permits are available within timeout' do
+        semaphore.drain_permits
+        Thread.new { sleep 0.1 && semaphore.release }
+        result = semaphore.try_acquire(1, 1)
+        expect(result).to be_truthy
+      end
+
+      it 'returns false on timeout' do
+        semaphore.drain_permits
+        result = semaphore.try_acquire(1, 0.1)
+        expect(result).to be_falsey
+      end
+    end
+  end
+
+  describe '#reduce_permits' do
+    it 'raises ArgumentError if reducing by negative number' do
+      expect {
+        semaphore.reduce_permits(-1)
+      }.to raise_error(ArgumentError)
+    end
+
+    it 'reduces permits below zero' do
+      semaphore.reduce_permits 1003
+      expect(semaphore.available_permits).to eq -1000
+    end
+
+    it 'reduces permits' do
+      semaphore.reduce_permits 1
+      expect(semaphore.available_permits).to eq 2
+      semaphore.reduce_permits 2
+      expect(semaphore.available_permits).to eq 0
+    end
+  end
+end
+
+module Concurrent
+  describe MutexSemaphore do
+    it_should_behave_like :semaphore
+
+    context 'spurious wake ups' do
+      subject { described_class.new(1) }
+
+      before(:each) do
+        def subject.simulate_spurious_wake_up
+          @mutex.synchronize do
+            @condition.signal
+            @condition.broadcast
+          end
+        end
+        subject.drain_permits
+      end
+
+      it 'should resist to spurious wake ups without timeout' do
+        @expected = true
+        # would set @expected to false
+        Thread.new { @expected = subject.acquire }
+
+        sleep(0.1)
+        subject.simulate_spurious_wake_up
+
+        sleep(0.1)
+        expect(@expected).to be_truthy
+      end
+
+      it 'should resist to spurious wake ups with timeout' do
+        @expected = true
+        # sets @expected to false in another thread
+        t = Thread.new { @expected = subject.try_acquire(1, 0.3) }
+
+        sleep(0.1)
+        subject.simulate_spurious_wake_up
+
+        sleep(0.1)
+        expect(@expected).to be_truthy
+
+        t.join
+        expect(@expected).to be_falsey
+      end
+    end
+  end
+
+  if TestHelpers.jruby?
+    describe JavaSemaphore do
+      it_should_behave_like :semaphore
+    end
+  end
+
+  describe Semaphore do
+    if jruby?
+      it 'inherits from JavaSemaphore' do
+        expect(Semaphore.ancestors).to include(JavaSemaphore)
+      end
+    else
+      it 'inherits from MutexSemaphore' do
+        expect(Semaphore.ancestors).to include(MutexSemaphore)
+      end
+    end
+  end
+end