The Colts did not disclose terms, but Castonzo will sign a two-year extension worth $33 million, according to ESPN and multiple reports.
Castonzo, who turns 32 on Aug. 9, contemplated retirement at the end of last season but notified Colts general manager Chris Ballard last month about his desire to play a 10th NFL season.
Durability is not an issue for Castonzo, who has only missed 12 games while starting the other 132 in which he's appeared since the Colts selected him in the first round of the 2011 draft.
His decision to continue playing means the Colts will not have to pursue a new starter this offseason. It also means the Colts will return the same offensive line that started all 16 games last season, barring any injuries. That group helped the Colts finish seventh in the NFL in rushing while giving up 32 sacks last season.
"Anthony has played a lot of great football, and he's been a great Colt," Ballard said last month. "He's going to continue to be a great Colt. I just wanted to give him his time [to decide]."