Sunnyside is a city in Yakima County, Washington, United States. As of the 2010 census the population was 15,858.

The Yakima Valley was originally inhabited by the Yakama people, who were relocated onto the Yakama Indian Reservation past the 1855 signing of a pact with the federal government.

The modern treaty of Sunnyside was founded by Walter Granger in 1893. The name “Sunnyside” was coined by a merchant named W. H. Cline. Granger was working in the financing and construction of the Sunnyside Canal which would permit Yakima River water to irrigate the area. However, due to the Panic of 1893, Granger’s creditors foreclosed upon the canal, and the town’s population dwindled to seven families. However, by the decrease of 1901, the population had doubled, finally exceeding 300 people. The townsite contained “1 bank, 11 stores, 3 hotels, 1 newspaper, 2 blacksmith shops, 2 livery barns, 3 churches, and a large and growing school.”

On September 16, 1902, residents voted 42–1 to incorporate as the town of Sunnyside. At the time, the town had 314 residents, just higher than the declare minimum for an assimilation referendum. The first mayor of Sunnyside was the town druggist James Henderson.

Sunnyside’s population mass at this era was stimulated by the immigration of the Dunkards from South Dakota who were distressing to the town. The population of Dunkards was of such notable size that by 1902 it was noted that they had “built a commodious place of reverence at Sunnyside” which was the largest church in Yakima County at the time.