William Plumer (June 25, 1759 – December 22, 1850) was an American lawyer, Baptist lay preacher, and politician from Epping, New Hampshire.

Frequent ill health left him unsuited for military service during the American Revolution or life as a farmer, and after a religious conversion experience in his late teens, Plumer was trained as a Baptist exhorter (a lay preacher).

Later deciding on a legal career, he studied law with attorneys Joshua Atherton of Amherst and John Prentice of Londonderry.

While studying under Atherton, his fellow law clerks included William Coleman, who remained a lifelong friend.

Recalling, in 1827, his involvement in the secession scheme, Plumer said, "This was, I think, the greatest political error of my life: & would, had it been reduced to practise [sic], instead of releiving [sic], destroyed New England.... Fortunately for my own reputation the erroneous opinion I formed produced no bitter fruits to myself or my country.

Some accounts say that it was to ensure that George Washington remained the only US president to be unanimously chosen by the Electoral College, but others assert that he was instead calling attention to his friend Adams as a potential future presidential candidate or was protesting against the "wasteful extravagance" of the Monroe administration.