The deal is worth $28 million, according to the San Diego Union-Tribune.
Dunlap, who was set to enter free agency on March 10, just completed his sixth season in the league.
Last season marked the first time he has started all 16 games.
“I am glad to be back with the Chargers organization,” Dunlap said. “I can’t say thank you enough to Dean Spanos, as well as Tom Telesco, Coach McCoy and Coach Joe D (D’Alessandris) for helping this day to arrive. It’s an honor and a blessing and I’m ready to get back to work.”
Prior to joining San Diego on a two-year, $3.7 million deal in 2013, Dunlap played four seasons for the Philadelphia Eagles.
Last season the Chargers won nine games and finished in third place in the AFC West.