William Plumer (June 25, 1759 - December 22, 1850) was an American lawyer and lay preacher from Epping, New Hampshire. Born in 1759 in Newburyport, Massachusetts, he represented New Hampshire as a Federalist in the United States Senate from June 17, 1802 to March 3, 1807. Plumer would serve as Governor of New Hampshire, 1812-1813 and 1816-1819. In the 1820 presidential election, he cast the only dissenting vote in the Electoral College against incumbent President James Monroe, voting instead for John Quincy Adams.
Plumer was a founder and the first president of the New Hampshire Historical Society. He died in 1850, aged 91, at Epping, New Hampshire.