Sunnyside is a city in Yakima County, Washington, United States. The population was 16,375 at the 2020 census.

Up through the forward portion of the 19th century, the part of the Yakima Valley where Sunnyside is now located was inhabited by the “Taptat-ħlama” (or ″People at the rapids”). These people hunted and fished along Yakima River from the mouth of Satus Creek (contained in present-day Satus snappishly southwest of Sunnyside) to gift Kiona, with a key fishery at close present-day Prosser.

Several tribes in the region were relocated onto the Yakama Indian Reservation similar to the 1855 signing of a settlement with the federal government. However, the Yakima War lingered until 1858, with Chief Kamiakin fighting upon until the Battle of Four Lakes in 1858.

The modern harmony of Sunnyside was founded by Walter Granger in 1893. The name was coined by a merchant named W. H. Cline. Granger was full of life in the financing and construction of the Sunnyside Canal which would have allowed Yakima River water to irrigate the area. However, due to the Panic of 1893, Granger’s creditors foreclosed on the canal, and the town’s population dwindled to seven families. By the decrease of 1901, the population had doubled, finally higher than 300 people. The site contained “1 bank, 11 stores, 3 hotels, 1 newspaper [the Sunnyside Sun, still publishing in 2020], 2 blacksmith shops, 2 livery barns, 3 churches, and a large and growing school”.