William Plumer Jr. (February 9, 1789 - September 18, 1854) was a U.S. Representative from New Hampshire, son of William Plumer.
Born in Epping, Plumer attended Phillips Exeter Academy, Exeter, New Hampshire. He graduated from Harvard University, Cambridge, Massachusetts, in 1809, and started his career as a lawyer in private practice and as an author.He was United States commissioner of loans in 1816 and 1817.
Plumer served as member of the New Hampshire House of Representatives in 1818 and as a member of the New Hampshire Senate from 1827 to 1828.
Plumer was elected as a Democratic-Republican to the Sixteenth and Seventeenth Congresses and reelected as an Adams-Clay Republican to the Eighteenth Congress (March 4, 1819 – March 3, 1825). He served as chairman of the Committee on the Judiciary (Seventeenth Congress). He was also a close friend and confidant of John Quincy Adams.
He served as delegate to the New Hampshire state constitutional convention in 1850.
Plumer died in Epping, New Hampshire, September 18, 1854. He was interred in the family burial ground on his father's estate near Epping.