Naval jack of the United States from September 11, 2002 to June 4, 2019; designed in the late 19th century. The flag consists of a rattlesnake superimposed across 13 alternating red and white stripes with the motto, "Don't Tread On Me" (without apostrophe).
First Navy Jack of the United States as (supposedly) used by the Continental Navy from October 13, 1775 through December 31, 1776.  In 1980, the Secretary of the Navy directed the commissioned ship in active status having the longest total period in active status to display the rattlesnake jack in place of the union jack until decommissioned or transferred to inactive status.  Since September 11, 2002, the flag has been flown by the United States Navy for the duration of the "Global War on Terrorism."  
This image is based on an image at the World Flag Database, with the color Red from Image:Flag of the United States.svg.

For the previous and current 50-star flag (still used as a "government jack" in some cases), see File:US Naval Jack.svg.
Naval Historical Center; vectorization copied from original upload of File:Naval jack of the United States.svg
U.S. Navy / User:Denelson83 (vectorization)