Rigby is a city in and the county seat of Jefferson County, Idaho, United States. The population was 5,038 at the 2020 census, up from 3,945 …Rigby is a city in and the county seat of Jefferson County, Idaho, United States. The population was 5,038 at the 2020 census, up from 3,945 in 2010. Native Americans from the Shoshone, Bannock Blackfeet, and Nez Perce tribes were the first inhabitants of the region.