Declo is a city in Cassia County, Idaho, United States. It is part of the Burley, Idaho Micropolitan Statistical Area. The population was 343 at the 2010 census. The world's largest potato processing company was founded in 1923 near Declo by a 14-year-old entrepreneur named J.R. Simplot. The corporate headquarters for the J. R. Simplot Company are now located in Boise.
Declo was originally named Marshfield. Its name was changed about 1916.
Declo High School's first graduating class graduated in 1927. The school's colors were green and white until about 1930, when they were changed to orange, black, and white.