"The opportunity to return to LSU is truly unique," Pelini said in a statement. "Culturally, with my prior experience at LSU, I know it is a great fit for me. The chance to work with coach (Ed) Orgeron, the ability to take charge of the Tigers' defense is something that I'm extremely excited about. All of that in a place that both my family and I immensely enjoyed when we were there before is very exciting for us. We are very honored and looking forward to this next chapter."
Pelini, 52, previously served as LSU's defensive coordinator from 2005-07, helping the school win the national title in his last year.
Dave Aranda held the role after Pelini's departure. The position was then vacated after Aranda assumed head coaching duties at Baylor following the 2019 season.
Pelini led Youngstown State to a 6-6 record last year after been named head coach in December 2014. He also spent seven seasons at Nebraska in the same capacity (2008-14). Overall, he holds a 100-55 career record as a bench boss.