John Rochester Thomas (June 18, 1848 – August 28, 1901) was an American architect credited in his time with being the nation's most prolific designer of public and semi-public buildings.

Thomas was born in Rochester, New York on June 18, 1848, where he was educated in the city's schools until 1862, when his father's business failure obliged him to seek employment.

After spending some time studying the architecture of European countries, Thomas entered professional practice in Rochester in 1868.

In 1874 John Adams Dix, governor of New York, appointed him architect and sole commissioner for the erection of the state reformatory at Elmira, under a special law.

In the combined armories of the 71st Regiment and 2d Battery (1893) he accomplished a feat never before attempted — the construction of two drill rooms, one above the other, free from all columns, 150 by 200 feet (46 by 61 metres) in area.

In rebuilding the New York Stock Exchange in 1886, Thomas successfully used an iron plate girder 102 feet (31 m) long in order to dispense with columns in the large board room, against the judgment of other experts.

In the Hays Building on Maiden Lane he first used the cantilever girder construction for distributing the load on the foundations, a system thereafter very much in vogue.

This action led to a proposal for the erection of a new Hall of Records on an adjoining site, budgeted at $5,000,000, intended in its construction and art details to be equal to the best that could be produced.