There are 7 US Post Offices in Jefferson County serving a total of 25,586 residents equating to 3,655 residents per post office. It's estimated that approximately 40,838 packages are handled across the 7 county post offices annually. Jefferson County is a county located in the U.S. state of Idaho. As of the 2010 census, the population was 26,140. The county seat and largest city is Rigby. The county was established in 1913 and named after Thomas Jefferson, the third U.S. President. Jefferson County is part of the Idaho Falls, ID Metropolitan Statistical Area."