John Gallagher Montgomery (June 27, 1805 – April 24, 1857) was a lawyer who represented Pennsylvania in the U.S. Congress briefly in 1857.

Montgomery was born in Northumberland, Pennsylvania on June 27, 1805. After studying under a private tutor, he graduated from Washington College (now Washington and Jefferson College) in Washington, Pennsylvania, in 1824. He then 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 poisoning related to poor sanitation.

Montgomery returned home ill, and died at Danville five weeks later. He was interred at the Episcopal Cemetery in Danville. A Cenotaph was erected in his honor at the Congressional Cemetery in Washington, D.C.

• List of United States Congress members who died in office (1790–1899)
• List of United States Congress members killed or wounded in office