William N. Roach (September 25, 1840 - September 7, 1902) was a United States Senator from North Dakota. Born in Washington, D.C., he attended the public schools, Gonzaga College High School, and Georgetown University. He was a clerk in the quartermaster's department during the Civil War. He moved to Dakota Territory in 1879 and settled in Larimore; he was interested in mail contracts for several years and was a member of the Territorial house of representatives in 1885.
Roach was an unsuccessful Democratic candidate for governor at the first State election in 1889 and again in 1891. He was elected to the U.S. Senate and served from March 4, 1893, to March 3, 1899; he was an unsuccessful candidate for reelection and discontinued active business pursuits and lived in retirement in Washington, D.C. He died in New York City on September 7, 1902; interment was in the Congressional Cemetery, Washington, D.C.