diff --git a/lib/bson.rb b/lib/bson.rb
index 15a54f373..4e5b7f778 100644
--- a/lib/bson.rb
+++ b/lib/bson.rb
@@ -21,20 +21,25 @@
 # @since 0.0.0
 module BSON
 
-  # Create a new object id from a string using ObjectId.from_string
+  # Interprets the argument as an ObjectId. If given a BSON::ObjectId, this method
+  # returns it directly without creating a new one. Otherwise, the argument is
+  # interpreted as a string and parsed accordingly.
   #
   # @example Create an object id from the string.
-  #   BSON::ObjectId(id)
+  #   BSON::ObjectId('...')
   #
-  # @param [ String ] string The string to create the id from.
+  # @param [ String | BSON::ObjectId ] string_or_object_id The string to create the id from,
+  #   or the existing object id to return.
   #
   # @raise [ BSON::Error::InvalidObjectId ] If the provided string is invalid.
   #
-  # @return [ BSON::ObjectId ] The new object id.
+  # @return [ BSON::ObjectId ] The new or existing object id.
   #
   # @see ObjectId.from_string
-  def self.ObjectId(string)
-    self::ObjectId.from_string(string)
+  def self.ObjectId(string_or_object_id)
+    return string_or_object_id if string_or_object_id.is_a?(self::ObjectId)
+
+    self::ObjectId.from_string(string_or_object_id)
   end
 
   # Constant for binary string encoding.
diff --git a/spec/bson_spec.rb b/spec/bson_spec.rb
index d849b7158..3f304d173 100644
--- a/spec/bson_spec.rb
+++ b/spec/bson_spec.rb
@@ -19,11 +19,23 @@
 
   describe ".ObjectId" do
 
-    let(:string) { "4e4d66343b39b68407000001" }
+    context 'when given a string' do
+      let(:string) { "4e4d66343b39b68407000001" }
 
-    it "returns an BSON::ObjectId from given string" do
-      expect(described_class::ObjectId(string)).to be_a BSON::ObjectId
-      expect(described_class::ObjectId(string)).to eq BSON::ObjectId.from_string(string)
+      it "returns a BSON::ObjectId from given string" do
+        expect(described_class::ObjectId(string)).to be_a BSON::ObjectId
+        expect(described_class::ObjectId(string)).to eq BSON::ObjectId.from_string(string)
+      end
+    end
+
+    context 'when given an object id' do
+      let(:object_id) do
+        described_class::ObjectId.new
+      end
+
+      it 'returns the same object' do
+        expect(described_class::ObjectId(object_id)).to be(object_id)
+      end
     end
   end