The 38-year-old Sirmon was at USC for two seasons and was the associate head coach, recruiting coordinator and linebackers coach. He replaces Manny Diaz, who left to be defensive coordinator at Miami.
Mississippi State also announced on Friday that it has hired Terrell Buckley to be its secondary coach and recruiting coordinator. The 43-year-old comes from Louisville, where he was the cornerbacks coach.
David Turner, who is in his seventh season with Mississippi State, has been named assistant head coach. He will continue to be the team's defensive line coach.