The University of Idaho will drop from the Football Bowl Subdivision level to the Football Championship Subdivision. The Vandals will become the first school to make the drop.
Idaho President Chuck Staben made the announcement Thursday.
"Our relevance will be complemented by our football program, not defined by it," Staben said.
The Vandals will return to the Big Sky, where they played from 1963 to '95. The conference already is home to Idaho's other athletic teams.
The Sun Belt Conference announced March 1 that it would drop Idaho and New Mexico State, who are both midway through four-year associate memberships, after the 2017 season. At the time, Staben said Idaho's offer from the Big Sky would have to be accepted by May 4.
Idaho has struggled as an FBS program, averaging 3.7 wins during its 20-year run but only 2.9 per season since 2000. It would face an uncertain future and the prospect of being an independent for the second time if it remained in the FBS.
Idaho, along with New Mexico State, was a member of the Western Athletic Conference until the WAC disbanded in 2012. Both schools spent the 2013 season as Division I independents before beginning a four-year term as Sun Belt football-only members in 2014.
In its two seasons in the Sun Belt, Idaho was 5-18 overall and 4-12 in league play. In coach Paul Petrino's three seasons at Idaho, the Vandals are 6-39. Idaho's last winning season was in 2009, when it went 8-5.
To return to the FCS, Idaho would have to decrease its football scholarships from 85 to 63, but it's a level at which the program thrived previously, winning eight Big Sky titles.
The Big Sky has 13 football-playing members: Cal Poly, Eastern Washington, Idaho State, Montana, Montana State, North Dakota, Northern Arizona, Northern Colorado, Portland State, Sacramento State, Southern Utah, UC Davis and Weber State.
If it increases to 14, the Big Sky could split into two seven-team divisions.
The Associated Press contributed to this report.