Pendergast, 48, was inside linebackers coach for the San Francisco 49ers this season.
“We are very excited to announce the return of Clancy Pendergast as our defensive coordinator,” Helton said in a statement released by the school. “I have had the good fortune to watch Clancy lead our defense firsthand and was blown away by his knowledge of the game as well as his teaching abilities.
"His success at both the NFL and college level makes it easy to see why he is one of the top defensive coaches in the game today. His aggressive style of defensive play is something I know our players as well as the Trojan Family will enjoy.”
Pendergast replaces Justin Wilcox, who was among four assistants fired by Helton after the Pac-12 Conference title game.
This will be Pendergast’s third stint with the Trojans.
He was a USC volunteer defensive assistant in 1992 before embarking on a career that included stints at Oklahoma and in the NFL with the Houston Oilers, Dallas Cowboys, Cleveland Browns, Arizona Cardinals and Kansas City Chiefs.
Pendergast was defensive coordinator at California for three seasons before Lane Kiffin hired him as USC’s defensive coordinator in 2013 after Monte Kiffin was forced out.
Lane Kiffin was fired after five games of the 2013 season, but Pendergast, who also coached defensive backs, headed a defense that finished a tumultuous season ranked 13th nationally.
The Trojans, however, did not play Oregon or Washington, which ranked first and second in offense, respectively, in the Pac-12 Conference. The Trojans also gave up 62 points to Arizona State in a defeat that led to Kiffin’s firing.
Pendergast is the fourth coach hired since the Trojans’ season-ending loss to Wisconsin in the Holiday Bowl.
Tyson Helton, Clay Helton’s younger brother, was hired as running backs coach and passing game coordinator, Neil Callaway as offensive line coach and John Baxter as special teams coordinator.
Helton had previously promoted receivers coach Tee Martin to offensive coordinator.
Marques Tuiasosopo, quarterbacks coach for the second half of the season, reportedly is headed to UCLA to fill the same position. Tyson Helton is expected to move to quarterbacks coach.
Johnny Nansen coached running backs and special teams the last two seasons. Helton has not announced what role Nansen will fill moving forward.
Helton has said he would have all or most of his staff in place by mid-January in anticipation of the final recruiting push toward national signing day in February.