John Semer Farnsworth (August 13, 1893 – November 10, 1952) was a United States Navy officer who was convicted of spying for Japan during the 1930s.

The Naval Academy yearbook described him as "daring and reckless", further stating that if Farnsworth lived in the days of the old navy, he "would have been famous for his desperate deeds and hairbreadth escapes".

After marrying a society woman, Farnsworth went heavily into debt, and borrowed money from an enlisted man, which he refused to repay.

He passed his information to his handlers, Commander Yoshiyuki Ichimiya, assistant Naval attaché at the Japanese Embassy from October 1932 to December 1934, and Lt.

Despite his disgraceful exit from Naval service, Farnsworth still had enough social grace to make him acceptable in the best Washington society.

Once, feigning drunkenness and pretending that he was a commander, he boarded a destroyer at Annapolis, tricked an ensign into giving him maneuver data, rushed back to the Japanese Embassy, had them photostatted, and returned them the next day.

However, when Farnsworth stole a confidential Navy manual, The Service of Information and Security, which contained plans for battle information and tactics that were gathered from field maneuvers and tested by high-ranking naval officers, alarm bells were raised, and the Office of Naval Intelligence (ONI) was called upon to investigate its disappearance.

Further investigation revealed that he had borrowed code and signal books and had been asking questions about tactics, new ship designs, and weapons.

Finally, the wife of a high-ranking officer living in Annapolis complained to the ONI that Farnsworth was pushing her to allow him to read official documents.

Faced with a sudden drop in income, and somehow having got wind that investigators were closing in on him, he approached Fulton Lewis Jr., the Washington correspondent for the Hearst newspapers, in early 1936.

He proposed to Lewis that he would write a series of articles entitled: "How I was a Spy in the American Navy for the Japanese Government" for $20,000 in an apparent effort to convince him that he was a double agent.

His lawyer, in turn, asked the court-martial commission to have Ichimiya and Yamaki testify in Farnsworth's defence through the American Consul General in Tokyo.

However, Japan refused the request, citing Japanese law prohibiting military officers from being compelled to answer questions in a foreign country.

On February 27, 1937, John Semer Farnsworth was sentenced to four to twelve years in prison for conspiring "to communicate and transmit to a foreign government—to wit Japan—writings, code books, photographs and plans relating to the national defense with the intent that they should be used to the injury of the United States".