William Plumer Jr. (February 9, 1789 – September 18, 1854) was an American politician, attorney, and author from New Hampshire.

He attended Phillips Exeter Academy and graduated from Harvard College in 1809.

Plumer then studied law with his father, attained admission to the bar in 1812, and commenced practice in Epping.

In 1816, Plumer was appointed a United States loan commissioner for New Hampshire, and he served until 1817.

Plumer authored several works in his later years, including a biography of his father that was published in 1857 and a book of religious-themed poetry, 1846's Lyrica Sacra.