Rexburg is a city in Madison County, Idaho, United States. The population was 39,409 at the 2020 census. The city is the county seat of Madison County and its largest city. Rexburg is the principal city of the Rexburg micropolitan area, which includes Fremont and Madison counties. The city is home to Brigham Young University–Idaho (BYU-Idaho), a private institution operated by the Church of Jesus Christ of Latter-day Saints.