diff --git a/CHANGELOG.md b/CHANGELOG.md
index cb7c0980..7ccc2478 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Features
+
+- Add support for custom headers in API requests via `:headers` option parameter. Useful for DocuSign-specific headers like `X-DocuSign-Edit` required for locked envelope operations.
+
 ### Bug Fixes
 
 - Fix custom opts in API requests - change `:query` to `:params` to match Req API requirements
diff --git a/lib/docusign/request_builder.ex b/lib/docusign/request_builder.ex
index dd8dd692..f4ed3481 100644
--- a/lib/docusign/request_builder.ex
+++ b/lib/docusign/request_builder.ex
@@ -54,13 +54,40 @@ defmodule DocuSign.RequestBuilder do
   - `definitions` (Map) - Map of parameter name to parameter location.
   - `options` (KeywordList) - The provided optional parameters
 
+  ### Special Parameters
+
+  - `:headers` - A map of custom headers to add to the request. These will be merged
+    with the default headers. This is useful for DocuSign-specific headers like
+    `X-DocuSign-Edit` which are required for certain operations on locked envelopes.
+
   ### Returns
 
   Map
+
+  ### Examples
+
+      # Add custom headers for locked envelope operations
+      optional_params = %{body: :body}
+      opts = [
+        body: envelope_data,
+        headers: %{"X-DocuSign-Edit" => ~s({"LockToken": "abc123", "LockDurationInSeconds": "600"})}
+      ]
+      add_optional_params(request, optional_params, opts)
+
   """
   @spec add_optional_params(map(), %{optional(atom()) => atom()}, keyword()) :: map()
   def add_optional_params(request, _, []), do: request
 
+  def add_optional_params(request, definitions, [{:headers, custom_headers} | tail]) when is_map(custom_headers) do
+    # Handle custom headers specially - merge them with existing headers
+    request =
+      Enum.reduce(custom_headers, request, fn {key, value}, acc ->
+        add_param(acc, :headers, key, value)
+      end)
+
+    add_optional_params(request, definitions, tail)
+  end
+
   def add_optional_params(request, definitions, [{key, value} | tail]) do
     case definitions do
       %{^key => location} ->
diff --git a/test/docusign/custom_headers_integration_test.exs b/test/docusign/custom_headers_integration_test.exs
new file mode 100644
index 00000000..e9155e06
--- /dev/null
+++ b/test/docusign/custom_headers_integration_test.exs
@@ -0,0 +1,243 @@
+defmodule DocuSign.CustomHeadersIntegrationTest do
+  use ExUnit.Case, async: false
+
+  alias DocuSign.Api.EnvelopeLocks
+  alias DocuSign.OAuth.Fake
+
+  setup do
+    bypass = Bypass.open()
+
+    # Store original config for cleanup
+    original_hostname = Application.get_env(:docusign, :hostname)
+    original_client_id = Application.get_env(:docusign, :client_id)
+    original_user_id = Application.get_env(:docusign, :user_id)
+    original_private_key_file = Application.get_env(:docusign, :private_key_file)
+    original_oauth_implementation = Application.get_env(:docusign, :oauth_implementation)
+
+    # Configure the app to use our test server
+    Application.put_env(:docusign, :hostname, "localhost:#{bypass.port}")
+    Application.put_env(:docusign, :client_id, "test_client_id")
+    Application.put_env(:docusign, :user_id, "test_user_id")
+    Application.put_env(:docusign, :private_key_file, "test/support/test_key")
+    Application.put_env(:docusign, :oauth_implementation, Fake)
+
+    # Start ClientRegistry if not already running with Fake implementation
+    case Process.whereis(DocuSign.ClientRegistry) do
+      nil ->
+        {:ok, _} = DocuSign.ClientRegistry.start_link(oauth_impl: Fake)
+
+      pid ->
+        # Stop existing registry and start with Fake implementation
+        Process.exit(pid, :kill)
+        {:ok, _} = DocuSign.ClientRegistry.start_link(oauth_impl: Fake)
+    end
+
+    # Add cleanup function
+    on_exit(fn ->
+      # Restore original config
+      if original_hostname do
+        Application.put_env(:docusign, :hostname, original_hostname)
+      else
+        Application.delete_env(:docusign, :hostname)
+      end
+
+      if original_client_id do
+        Application.put_env(:docusign, :client_id, original_client_id)
+      else
+        Application.delete_env(:docusign, :client_id)
+      end
+
+      if original_user_id do
+        Application.put_env(:docusign, :user_id, original_user_id)
+      else
+        Application.delete_env(:docusign, :user_id)
+      end
+
+      if original_private_key_file do
+        Application.put_env(:docusign, :private_key_file, original_private_key_file)
+      else
+        Application.delete_env(:docusign, :private_key_file)
+      end
+
+      if original_oauth_implementation do
+        Application.put_env(:docusign, :oauth_implementation, original_oauth_implementation)
+      else
+        Application.delete_env(:docusign, :oauth_implementation)
+      end
+    end)
+
+    {:ok, bypass: bypass}
+  end
+
+  describe "custom headers support" do
+    test "X-DocuSign-Edit header is sent in PUT requests for locked envelopes", %{
+      bypass: bypass
+    } do
+      # Setup - Get connection
+      {:ok, conn} = DocuSign.Connection.get("test_user_id")
+
+      account_id = "17035828"
+      envelope_id = "test-envelope-123"
+      lock_token = "abc123-lock-token"
+      lock_duration = "600"
+
+      lock_edit_header =
+        Jason.encode!(%{
+          "LockDurationInSeconds" => lock_duration,
+          "LockToken" => lock_token
+        })
+
+      # Mock the envelope lock update endpoint
+      Bypass.expect_once(
+        bypass,
+        "PUT",
+        "/restapi/v2.1/accounts/#{account_id}/envelopes/#{envelope_id}/lock",
+        fn conn ->
+          # CRITICAL: Verify the X-DocuSign-Edit header was sent!
+          headers = Map.new(conn.req_headers)
+
+          assert headers["x-docusign-edit"] == lock_edit_header,
+                 "Expected X-DocuSign-Edit header to be present with lock token"
+
+          # Read and verify body
+          {:ok, body, conn} = Plug.Conn.read_body(conn)
+          request_body = Jason.decode!(body)
+
+          # Verify request structure
+          assert request_body["lockDurationInSeconds"]
+
+          # Return success response
+          response = %{
+            "lockDurationInSeconds" => lock_duration,
+            "lockToken" => lock_token,
+            "lockedByUser" => %{
+              "email" => "test@example.com",
+              "userName" => "Test User"
+            }
+          }
+
+          conn
+          |> Plug.Conn.put_resp_content_type("application/json")
+          |> Plug.Conn.resp(200, Jason.encode!(response))
+        end
+      )
+
+      # Act - Call the API with custom headers
+      lock_request_body = %{
+        "lockDurationInSeconds" => lock_duration
+      }
+
+      result =
+        EnvelopeLocks.lock_put_envelope_lock(
+          conn,
+          account_id,
+          envelope_id,
+          body: lock_request_body,
+          headers: %{"X-DocuSign-Edit" => lock_edit_header}
+        )
+
+      # Assert - Verify success
+      assert {:ok, lock_info} = result
+      assert lock_info.lockToken == lock_token
+      assert lock_info.lockDurationInSeconds == lock_duration
+    end
+
+    test "multiple custom headers are sent correctly", %{bypass: bypass} do
+      {:ok, conn} = DocuSign.Connection.get("test_user_id")
+
+      account_id = "17035828"
+      envelope_id = "test-envelope-456"
+
+      custom_header_1_value = "custom-value-1"
+      custom_header_2_value = "custom-value-2"
+
+      # Mock endpoint that expects multiple custom headers
+      Bypass.expect_once(
+        bypass,
+        "PUT",
+        "/restapi/v2.1/accounts/#{account_id}/envelopes/#{envelope_id}/lock",
+        fn conn ->
+          headers = Map.new(conn.req_headers)
+
+          # Verify both custom headers are present
+          assert headers["x-custom-header-1"] == custom_header_1_value
+          assert headers["x-custom-header-2"] == custom_header_2_value
+
+          {:ok, body, conn} = Plug.Conn.read_body(conn)
+          Jason.decode!(body)
+
+          response = %{
+            "lockDurationInSeconds" => "300",
+            "lockToken" => "token123"
+          }
+
+          conn
+          |> Plug.Conn.put_resp_content_type("application/json")
+          |> Plug.Conn.resp(200, Jason.encode!(response))
+        end
+      )
+
+      # Call with multiple custom headers
+      result =
+        EnvelopeLocks.lock_put_envelope_lock(
+          conn,
+          account_id,
+          envelope_id,
+          body: %{"lockDurationInSeconds" => "300"},
+          headers: %{
+            "X-Custom-Header-1" => custom_header_1_value,
+            "X-Custom-Header-2" => custom_header_2_value
+          }
+        )
+
+      assert {:ok, _lock_info} = result
+    end
+
+    test "custom headers work with POST requests", %{bypass: bypass} do
+      {:ok, conn} = DocuSign.Connection.get("test_user_id")
+
+      account_id = "17035828"
+      envelope_id = "test-envelope-789"
+
+      custom_header_value = "post-custom-value"
+
+      # Mock the envelope lock creation endpoint
+      Bypass.expect_once(
+        bypass,
+        "POST",
+        "/restapi/v2.1/accounts/#{account_id}/envelopes/#{envelope_id}/lock",
+        fn conn ->
+          headers = Map.new(conn.req_headers)
+
+          # Verify custom header is present
+          assert headers["x-custom-header"] == custom_header_value
+
+          {:ok, body, conn} = Plug.Conn.read_body(conn)
+          Jason.decode!(body)
+
+          response = %{
+            "lockDurationInSeconds" => "600",
+            "lockToken" => "new-lock-token"
+          }
+
+          conn
+          |> Plug.Conn.put_resp_content_type("application/json")
+          |> Plug.Conn.resp(201, Jason.encode!(response))
+        end
+      )
+
+      # Call POST endpoint with custom headers
+      result =
+        EnvelopeLocks.lock_post_envelope_lock(
+          conn,
+          account_id,
+          envelope_id,
+          body: %{"lockDurationInSeconds" => "600"},
+          headers: %{"X-Custom-Header" => custom_header_value}
+        )
+
+      assert {:ok, lock_info} = result
+      assert lock_info.lockToken == "new-lock-token"
+    end
+  end
+end
diff --git a/test/docusign/request_builder_test.exs b/test/docusign/request_builder_test.exs
index c4d3dbbc..2a35e3a7 100644
--- a/test/docusign/request_builder_test.exs
+++ b/test/docusign/request_builder_test.exs
@@ -30,4 +30,92 @@ defmodule DocuSign.RequestBuilderTest do
       refute Map.has_key?(decoded, "approveTabs")
     end
   end
+
+  describe "custom headers support" do
+    test "adds custom headers to request" do
+      request = %{}
+      optional_params = %{}
+
+      opts = [
+        headers: %{
+          "X-DocuSign-Edit" => ~s({"LockToken": "abc123", "LockDurationInSeconds": "600"})
+        }
+      ]
+
+      result = RequestBuilder.add_optional_params(request, optional_params, opts)
+
+      assert %{headers: headers} = result
+      assert {"X-DocuSign-Edit", ~s({"LockToken": "abc123", "LockDurationInSeconds": "600"})} in headers
+    end
+
+    test "merges custom headers with existing headers" do
+      request = %{headers: [{"existing-header", "existing-value"}]}
+      optional_params = %{}
+
+      opts = [
+        headers: %{
+          "X-DocuSign-Edit" => ~s({"LockToken": "token123"})
+        }
+      ]
+
+      result = RequestBuilder.add_optional_params(request, optional_params, opts)
+
+      assert %{headers: headers} = result
+      assert {"X-DocuSign-Edit", ~s({"LockToken": "token123"})} in headers
+      assert {"existing-header", "existing-value"} in headers
+    end
+
+    test "supports multiple custom headers" do
+      request = %{}
+      optional_params = %{}
+
+      opts = [
+        headers: %{
+          "X-Another-Header" => "value3",
+          "X-Custom-Header" => "value2",
+          "X-DocuSign-Edit" => "value1"
+        }
+      ]
+
+      result = RequestBuilder.add_optional_params(request, optional_params, opts)
+
+      assert %{headers: headers} = result
+      assert {"X-DocuSign-Edit", "value1"} in headers
+      assert {"X-Custom-Header", "value2"} in headers
+      assert {"X-Another-Header", "value3"} in headers
+    end
+
+    test "works alongside other optional params" do
+      request = %{}
+      optional_params = %{body: :body}
+
+      body_data = %{test: "data"}
+
+      opts = [
+        body: body_data,
+        headers: %{"X-Custom-Header" => "custom-value"}
+      ]
+
+      result = RequestBuilder.add_optional_params(request, optional_params, opts)
+
+      assert %{body: body, headers: headers} = result
+      assert body == Jason.encode!(body_data)
+      assert {"X-Custom-Header", "custom-value"} in headers
+    end
+
+    test "ignores headers option if not a map" do
+      request = %{}
+      optional_params = %{}
+
+      opts = [
+        headers: "invalid-not-a-map"
+      ]
+
+      # Should not crash, just ignore the invalid headers
+      result = RequestBuilder.add_optional_params(request, optional_params, opts)
+
+      # Should not have added any headers
+      refute Map.has_key?(result, :headers)
+    end
+  end
 end