Prosser () is a city in and the county chair of Benton County, Washington, United States, along the Yakima River once only one zip code 99350. The population was 5,714 at the 2010 census.

Prosser was long house 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 Place 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 take in hand these duties, and never returned to the town that he founded.

Lewis Hinzerling built a flour mill at Prosser falls in 1887, encouraging further harmony of the area. The first irrigation canal was completed in 1893 by the Prosser Falls Land and Irrigation Company. Prosser was officially incorporated in 1899 like a population of 229 people.

In 1905, Benton County was carved out of the eastern portions of Yakima and Klickitat Counties. The additional town of Prosser was fixed as county seat. In 1907 a knack plant was other and began delivering electricity to the town. The following year, a new high school was built, followed a year higher by a telephone exchange. In 1910 the city time-honored a take over from the Andrew Carnegie for a public library.