Declo is a city in Cassia County, Idaho, United States. 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, Idaho. Declo apartments for rent comparison page shows most up to date rent information on Declo apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.