John Gallagher Montgomery (June 27, 1805 – April 24, 1857) was an American lawyer from Danville, Pennsylvania. He represented Pennsylvania in the U.S. Congress briefly in 1857.

John Montgomery was born in Northumberland, Pennsylvania. He studied under a private tutor, and graduated from Washington College (now Washington and Jefferson College) in Washington, Pennsylvania, in 1824. He studied law, was admitted to the bar in 1827 and commenced practice in Danville. He was a member of the Pennsylvania House of Representatives in 1855.

Montgomery was elected as a Democrat to the Thirty-fifth Congress and served until his death. He attended the inauguration dinner for President Buchanan at the National Hotel where he was reported to have been deliberately poisoned along with many other attendees. This incident is now known as National Hotel Disease, and is believed to have been caused by food poisoningrelated to bad sanitation. He returned home ill, and died at Danville five weeks later. Interment at Episcopal Cemetery in Danville. Cenotaph at Congressional Cemetery in Washington, D.C.