John Isaac De Graff (October 2, 1783 – July 26, 1848) was an American businessman and politician who served two non-consecutive term as a U.S. Representative from New York 1827 to 1829, and again from 1837 to 1839.

He was also a successful real estate investor and landlord, and owned several houses, farms, commercial buildings and pastures in and around Schenectady.

In the early 1800s De Graff served in the militia, and was appointed an Ensign in the Schenectady regiment commanded by Jacob Swits.

He pledged both private funds and his personal credit to the amount of $100,000 (about $1.1 million in 2015), and with this financing Thomas Macdonough was able to build and outfit the ships he used to defeat the British at the Battle of Plattsburgh.

He presented evidence to support his charge that the ballots of Schenectady's second ward had been tampered with, and that the fraud was sufficient to have affected the outcome, but the Assembly voted to seat Theodore W. Sanders, the Whig nominee.