Prosser is a city in and the county seat of Benton County, Washington, United States, along the Yakima River with only one zip code 99350. The population was 5,714 at the 2010 census.
Prosser was long home to Native Americans who lived and fished along the river. They called the area "Tap tut", meaning rapids.
Colonel William Farrand Prosser first surveyed the area in 1879, then claimed homestead in 1882. The Northern Pacific Railroad laid tracks through the area two years later. A town plat was filed by Colonel Prosser in 1885, and in 1886 he was elected Yakima County Auditor. He moved to North Yakima to attend to these duties, and never returned to the town that he founded.