The 25-year-old Boston played for new Arizona coach Steve Wilks in Carolina from 2014 to 2016.
Released in a salary cost-cutting move during the offseason by the Panthers last year, Boston bet on himself, signing a one-year, $900,000 deal to join the Chargers and had his best season as a professional. He played all 16 games and led the Chargers with five interceptions.
Boston also finished tied for second on the team with 79 tackles and developed into one of the vocal leaders on a defense that gave up just 17 points per game, ranked No. 3 in the NFL.
Boston played a team-high 986 defensive snaps for the Chargers and finished with eight pass breakups last season.
A fourth-round selection by the Panthers in the 2014 draft, Boston has recorded 187 combined tackles, 18 pass breakups and 8 interceptions in four NFL seasons.
ESPN's Eric D. Williams contributed to this report.