Leavitt is expected to assist Seminoles defensive coordinator Harlon Barnett by bringing another experienced voice to help with devising game plans.
"It's a great addition to our football staff, our football team, to get somebody of that quality and that experience in the defensive room, and someone Barnett and the other defensive coaches can lean on," Taggart told reporters Thursday. "It's a plus for our football team, and it's something that will help our football team."
Leavitt was the defensive coordinator in Taggart's only season as Ducks head coach in 2017.
Leavitt stayed behind when Taggart took the FSU job, hoping to become head coach of the Ducks, which instead went to Mario Cristobal.
Leavitt remained the Ducks' defensive coordinator last season, but he had a rocky relationship with Cristobal and left after the season with a $2.5 million buyout. Leavitt has been out of college coaching since.
"It was something we had talked about before," Taggart said of working again with Leavitt. "Picked up the phone, called, and he said, 'Absolutely.' The right timing, I guess.
"Whenever we can help our football team and help ourselves get better, then we're going to try and do that."
Florida State (1-1) allowed 1,040 yards in its first two games, relinquishing double-digit leads in both. The Seminoles rank 83rd in the country in yards per play allowed (5.56) and 124th in yards per game (520.0).
Leavitt was 95-57 in 13 seasons as head coach at South Florida, building a fledgling program into a regular bowl participant. He was fired in 2009 after an investigation by the school found he grabbed and hit a player, and then misled school officials.
He landed in the NFL with the San Francisco 49ers before returning to college as defensive coordinator at Colorado under Mike MacIntyre in 2015. Leavitt helped turn around a struggling defense and the Buffs won the Pac-12 South in 2016.
Taggart, who was the head coach at South Florida from 2016 to 2016, hired Leavitt at Oregon in 2017. After Taggart left Oregon, Leavitt was given a four-year contract that paid $1.7 million per season, making him one of the highest-paid coordinators in college football.
ESPN's Adam Rittenberg and The Associated Press contributed to this report.