Seto played at USC and coached linebackers and the secondary for the Trojans before joining Carroll's NFL staff in 2010.
Seto's promotion came a day after the Seahawks announced several staff moves involving former USC coaches and players.
Kris Richard was promoted from defensive backs coach to defensive coordinator, replacing Dan Quinn. Richard played defensive back at USC under Carroll and served on the Trojans coaching staff in 2008 and 2009.
Former Trojans and Seahawks linebacker Lofa Tatupu was hired as an assistant linebackers coach.
Carroll's oldest son, Brennan, was hired as an assistant offensive line coach. Brennan Carroll coached tight ends and was recruiting coordinator at USC before joining the staff at the University of Miami.