William D. Washburn (January 14, 1831-July 29, 1912) was an American politician. He served in both the United States House of Representatives and the United States Senate as a Republican from Minnesota. Three of his seven brothers became politicians: Elihu B. Washburne, Cadwallader C. Washburn, and Israel Washburn, Jr. He was also cousin of Dorilus Morrison, the first mayor of Minneapolis.
Washburn was born in Livermore, Maine. He moved to Minneapolis, Minnesota around 1857 to practice law and work for the Minneapolis Milling Company (founded by his brother Cadwallader). His business ventures in lumber and flour milling allowed him to amass a large fortune, and by the 1880s, he was among the wealthiest men in Minnesota. He built a mansion known as "Fair Oaks" in 1883. It was designed by E. Townsend Mix, who also designed Minneapolis's Metropolitan Building, and the outdoor landscape was laid out by Frederick Law Olmsted. The grounds included an artificial stream leading to a pond, a rustic footbridge, a greenhouse, and a carriage house. The home was demolished in 1924 to make way for a park, although the region is now part of the Washburn-Fair Oaks Historic District, which was added to the National Register of Historic Places in 1977.
He was elected to the House of Representatives in 1878 and served from March 4, 1879 to March 3, 1885. He was elected to the Senate in 1888 and served from March 4, 1889 to March 3, 1895.
He died in Minneapolis.