William Pierce Frye (September 2, 1830 – August 8, 1911) was an American politician from the U.S. state of Maine. Frye spent most of his political career as a legislator, serving in the Maine House of Representatives and U.S. House of Representatives before being elected to the U.S. Senate, where he served for 30 years and died in office. Frye was a member of the Frye political family, and was the grandfather of Wallace H. White, Jr. and the son of John March Frye. He was also a prominent member of the Peucinian Society tradition.

Frye was born in Lewiston, Maine, in Androscoggin County. He attended public schools there and graduated from Bowdoin College in Brunswick in 1850. Frye studied law and was later admitted to the bar. He began practicing in Rockland, Maine, in 1853 but later returned to Lewiston, and practiced law there. Frye played a role in founding Bates College in Lewiston and served as a longtime trustee of the College. Frye received a LL.D. from Bates in 1881.

Frye served in the Maine House of Representatives from 1861 to 1862 and again in 1867. He was later elected as the mayor of Lewiston, holding that position from 1866 to 1867, when he became the state attorney general. Frye left the attorney general post in 1869. He was elected as a Republican in 1870 to the U.S. House of Representatives. Frye served in the 42nd Congress and the five succeeding Congresses from March 4, 1871, to March 17, 1881, when he resigned after being elected Senator to fill the vacancy caused by the resignation of James G. Blaine. He served over 30 years in the Senate (March 18, 1881 – August 8, 1911), and was reelected in 1883, 1889, 1895, 1901, and 1907.

During his tenure in the Senate, Frye served as President pro tempore (54th – 62nd Congress, his resignation shortly before his death spawning a debate over filling the position) and chairman of the Rules Committee (47th – 49th Congress). Frye also was a member of the Commerce Committee (50th – 62nd Congress) and a member of the commission which met in Paris in September 1898 to adjust the Treaty of Paris between the United States and Spain, ending the Spanish - American War.

Frye died in Lewiston in 1911. He is interred in the Riverside Cemetery. The Sen. William P. Frye House near Bates College in Lewiston is on the National Historic Register.