USS K-8 (SS-39) was an K-class submarine of the United States Navy. Her keel was laid down by the Union Iron Works in San Francisco, California, under subcontract from Electric Boat Company of Groton, Connecticut. She was launched on 11 July 1914 sponsored by Mrs. John W. Lewis, wife of the first commanding officer, and commissioned on 1 December at Mare Island with Lieutenant John W. Lewis in command.