diff --git a/lib/process_shared/monitor.rb b/lib/process_shared/monitor.rb
index 51ea44a..e5f0bf5 100644
--- a/lib/process_shared/monitor.rb
+++ b/lib/process_shared/monitor.rb
@@ -9,7 +9,7 @@ module ProcessShared
     end
 
     def lock
-      if locked_by == ::Process.pid
+      if locked_by == current_process_and_thread
         @lock_count += 1
       else
         super
@@ -17,7 +17,7 @@ module ProcessShared
     end
 
     def unlock
-      if locked_by == ::Process.pid
+      if locked_by == current_process_and_thread
         if @lock_count > 0
           @lock_count -= 1
         else
diff --git a/lib/process_shared/mutex.rb b/lib/process_shared/mutex.rb
index 2b31d58..04efdeb 100644
--- a/lib/process_shared/mutex.rb
+++ b/lib/process_shared/mutex.rb
@@ -4,13 +4,14 @@ require 'process_shared/process_error'
 
 module ProcessShared
   # This Mutex class is implemented as a Semaphore with a second
-  # internal Semaphore used to track the locking process is tracked.
+  # internal Semaphore used to track the locking process and thread.
+  #
   # {ProcessError} is raised if either {#unlock} is called by a
-  # process different from the locking process, or if {#lock} is
-  # called while the process already holds the lock (i.e. the mutex is
-  # not re-entrant).  This tracking is not without performance cost,
-  # of course (current implementation uses the additional {Semaphore}
-  # and {SharedMemory} segment).
+  # process + thread different from the locking process + thread, or
+  # if {#lock} is called while the process + thread already holds the
+  # lock (i.e. the mutex is not re-entrant).  This tracking is not
+  # without performance cost, of course (current implementation uses
+  # the additional {Semaphore} and {SharedMemory} segment).
   #
   # The API is intended to be identical to the {::Mutex} in the core
   # Ruby library.
@@ -23,25 +24,25 @@ module ProcessShared
 
     def initialize
       @internal_sem = Semaphore.new
-      @locked_by = SharedMemory.new(:int)
+      @locked_by = SharedMemory.new(:uint64, 2)  # [Process ID, Thread ID]
 
       @sem = Semaphore.new
     end
 
     # @return [Mutex]
     def lock
-      if locked_by == ::Process.pid
-        raise ProcessError, "already locked by this process #{::Process.pid}"
+      if (p, t = current_process_and_thread) == locked_by
+        raise ProcessError, "already locked by this process #{p}, thread #{t}"
       end
 
       @sem.wait
-      self.locked_by = ::Process.pid
+      self.locked_by = current_process_and_thread
       self
     end
 
     # @return [Boolean]
     def locked?
-      locked_by > 0
+      locked_by != UNLOCKED
     end
 
     # Releases the lock and sleeps timeout seconds if it is given and
@@ -60,11 +61,11 @@ module ProcessShared
     # @return [Boolean]
     def try_lock
       with_internal_lock do
-        if @locked_by.get_int(0) > 0
+        if locked?
           false                 # was locked
         else
           @sem.wait             # should return immediately
-          self.locked_by = ::Process.pid
+          self.locked_by = current_process_and_thread
           true
         end
       end
@@ -72,11 +73,11 @@ module ProcessShared
 
     # @return [Mutex]
     def unlock
-      if (p = locked_by) != ::Process.pid
-        raise ProcessError, "lock is held by #{p} not #{::Process.pid}"
+      if (p, t = locked_by) != (cp, ct = current_process_and_thread)
+        raise ProcessError, "lock is held by process #{p}, thread #{t}: not process #{cp}, thread #{ct}"
       end
 
-      self.locked_by = 0
+      self.locked_by = UNLOCKED
       @sem.post
       self
     end
@@ -96,20 +97,32 @@ module ProcessShared
 
     protected
 
+    # @return [Array<(Fixnum, Fixnum)>]
+    #   If locked, IDs of the locking process and thread, otherwise +UNLOCKED+
     def locked_by
       with_internal_lock do
-        @locked_by.get_int(0)
+        @locked_by.read_array_of_uint64(2)
       end
     end
 
-    def locked_by=(val)
+    # @param [Array<(Fixnum, Fixnum)>] ary
+    #   Set the IDs of the locking process and thread, or +UNLOCKED+ if none
+    def locked_by=(ary)
       with_internal_lock do
-        @locked_by.put_int(0, val)
+        @locked_by.write_array_of_uint64(ary)
       end
     end
 
     def with_internal_lock(&block)
       @internal_sem.synchronize &block
     end
+
+    # @return [Array<(Fixnum, Fixnum)>]  IDs of the current process and thread
+    def current_process_and_thread
+      [::Process.pid, Thread.current.object_id]
+    end
+
+    # Represents the state of being unlocked
+    UNLOCKED = [0, 0].freeze
   end
 end
diff --git a/spec/process_shared/mutex_spec.rb b/spec/process_shared/mutex_spec.rb
index 31e167e..9a0fd7e 100644
--- a/spec/process_shared/mutex_spec.rb
+++ b/spec/process_shared/mutex_spec.rb
@@ -24,10 +24,37 @@ module ProcessShared
       ::Process.wait(pid)
     end
 
-    it 'raises exception when locked twice by same process' do
+    it 'raises exception when unlocked by other thread in same process' do
+      t = Thread.new do
+        @lock.lock
+        sleep 0.2
+        @lock.unlock
+      end
+
+      sleep 0.1
+      proc { @lock.unlock }.must_raise(ProcessError)
+
+      t.join
+    end
+
+    it 'raises exception when locked twice by same process and thread' do
       @lock.lock
       proc { @lock.lock }.must_raise(ProcessError)
       @lock.unlock
     end
+
+    it 'does not raise when locked by different threads on same process' do
+      t = Thread.new do
+        @lock.lock
+        sleep 0.2
+        @lock.unlock
+      end
+
+      sleep 0.1
+      @lock.synchronize { }
+
+      t.join
+    end
+
   end
 end