Ash, who was the defensive coordinator for Ohio State's 2014 national championship team -- where Herman served as the offensive coordinator -- will join the Longhorns' staff Wednesday.
"After taking a hard look at a lot of options at defensive coordinator, Chris was the one that continued to stand out," Herman said in a statement. "We couldn't be more excited what he'll add to our staff."
Ash and Herman have coached on the same staff twice before; the 2009 season at Iowa State was their first stint together.
"I have a great deal of respect for Coach Herman as a coach and a person," Ash said in a statement. "If I didn't believe in him, I wouldn't be making this move. I have complete confidence and faith and trust in his ability to lead and to coach, and I'm excited to be a part of this program."
Ash, 45, spent the past three-plus seasons at Rutgers where he went 8-32 as head coach and was fired after a 1-3 start this year. Before that he spent 16 years as an assistant with stops at Iowa State, San Diego State, Wisconsin, Arkansas and Ohio State.
In his five seasons as a defensive coordinator, Ash's defenses ranked in the top 25 nationally in total defense four times.
Herman, after a 7-5 regular season for the Longhorns, decided to change both coordinators, firing former defensive coordinator Todd Orlando and removing the offensive coordinator title from Tim Beck. Herman has yet to fill the offensive coordinator vacancy.
Ash will not coach in the upcoming Valero Alamo Bowl, where Texas will play Utah. Co-defensive coordinator and safeties coach Craig Naivar will lead the Longhorns' defense in the game.