diff --git a/lib/bson.rb b/lib/bson.rb
index 2148abfd4..20766bc9e 100644
--- a/lib/bson.rb
+++ b/lib/bson.rb
@@ -21,16 +21,18 @@
 # @since 0.0.0
 module BSON
 
-  # Create a new object id from a string using ObjectId.from_string
+  # Create a new object id from a string. If given a BSON::ObjectId object instead,
+  # this method returns that object id without creating a new one.
   #
   # @example Create an object id from the string.
   #   BSON::ObjectId(id)
   #
-  # @param [ String ] string The string to create the id from.
+  # @param [ String | BSON::ObjectId ] string 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)
diff --git a/lib/bson/object_id.rb b/lib/bson/object_id.rb
index cf3076339..25062672a 100644
--- a/lib/bson/object_id.rb
+++ b/lib/bson/object_id.rb
@@ -284,19 +284,23 @@ def from_data(data)
         object_id
       end
 
-      # Create a new object id from a string.
+      # Create a new object id from a string. If given a BSON::ObjectId object instead,
+      # this method returns that object id without creating a new one.
       #
       # @example Create an object id from the string.
       #   BSON::ObjectId.from_string(id)
       #
-      # @param [ String ] string The string to create the id from.
+      # @param [ String | BSON::ObjectId ] string 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.
       #
       # @since 2.0.0
       def from_string(string)
+        return string if string.is_a?(self)
+
         raise Error::InvalidObjectId, "'#{string}' is an invalid ObjectId." unless legal?(string)
 
         from_data([ string ].pack('H*'))
diff --git a/spec/bson/object_id_spec.rb b/spec/bson/object_id_spec.rb
index 42293e60b..2ff9e8aa9 100644
--- a/spec/bson/object_id_spec.rb
+++ b/spec/bson/object_id_spec.rb
@@ -326,6 +326,16 @@
         }.to raise_error(BSON::Error::InvalidObjectId)
       end
     end
+
+    context 'when given an object id' do
+      let(:object_id) do
+        described_class.new
+      end
+
+      it 'returns the same object' do
+        expect(described_class.from_string(object_id)).to be(object_id)
+      end
+    end
   end
 
   describe ".from_time" do
diff --git a/spec/bson_spec.rb b/spec/bson_spec.rb
index d849b7158..d9e3d030c 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 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)
+      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