He replaces Gus Malzahn, who was fired earlier this month after eight seasons with the Tigers.
The 44-year-old Harsin is 69-19 with three Mountain West titles in seven seasons at Boise State, his alma mater. Harsin was a top target for Auburn from the start of its coaching search, and more than 20 Power 5 schools have reached out to him over the years, a source told Rittenberg.
Malzahn’s firing came as a surprise after a 6-4 season and cost the school more than $21 million to buy out the former coach’s contract.
Auburn has been an up-and-down program, winning the national championship under Gene Chizik before going on a rapid decline.
The Tigers turned to Malzhan, the offensive coordinator for that Cam Newton-led team. Malzahn led Auburn to the SEC championship and into the national championship game in his first season, 2013. But the Tigers have lost at least four games in each of the seven seasons since.
Yahoo Sports first reported Auburn was narrowing its search to Harsin, and The Athletic was first to report the agreement.
The Associated Press contributed to this report.