diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index cae4ee7fde..25667af101 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Support :unless_exist in FileStore
+
+    *Michael Grosser*
+
 *   Fix `slice!` deleting the default value of the hash.
 
     *Antonio Santos*
diff --git a/activesupport/lib/active_support/cache/file_store.rb b/activesupport/lib/active_support/cache/file_store.rb
index 10d39463ec..5cd6065077 100644
--- a/activesupport/lib/active_support/cache/file_store.rb
+++ b/activesupport/lib/active_support/cache/file_store.rb
@@ -97,6 +97,7 @@ module ActiveSupport
 
         def write_entry(key, entry, options)
           file_name = key_file_path(key)
+          return false if options[:unless_exist] && File.exist?(file_name)
           ensure_cache_path(File.dirname(file_name))
           File.atomic_write(file_name, cache_path) {|f| Marshal.dump(entry, f)}
           true
diff --git a/activesupport/test/caching_test.rb b/activesupport/test/caching_test.rb
index 16d087ab9d..51007402a1 100644
--- a/activesupport/test/caching_test.rb
+++ b/activesupport/test/caching_test.rb
@@ -721,6 +721,13 @@ class FileStoreTest < ActiveSupport::TestCase
     assert @cache.exist?('baz')
     assert @cache.exist?('quux')
   end
+
+  def test_write_with_unless_exist
+    assert_equal true, @cache.write(1, "aaaaaaaaaa")
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
+    @cache.write(1, nil)
+    assert_equal false, @cache.write(1, "aaaaaaaaaa", unless_exist: true)
+  end
 end
 
 class MemoryStoreTest < ActiveSupport::TestCase