The athletic department said in a news release Sunday that Smith will pursue other career opportunities.
Head coach Derek Dooley said in a statement that Smith is a “dear friend” and thanked him for the hard work and passion he contributed to the program.
Lance Thompson, the linebackers coach for the last two years, will take over. Thompson previously coached defensive lines at Georgia Tech, Alabama and LSU.
Sirmon spent seven seasons as a linebacker for the NFL’s Tennessee Titans. Last season he helped coach the Volunteers’ safeties as a graduate assistant.