Sunnyside is a city in Yakima County, Washington, United States. As of the 2010 Census the population was 15,858.
On September 16, 1902, residents voted 42 to one to incorporate as the town of Sunnyside. By state law a town needed to have 300 citizens in order to legally incorporate. With 314 residents, Sunnyside was just eligible to legally vote for incorporation.
The first mayor of Sunnyside was the town druggist James Henderson.