Declo is a city in Cassia County, Idaho, United States. It is part of the Burley, Idaho Micropolitan Statistical Area. The population was 338 at the 2000 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.