"Left tackles like Anthony Castonzo are hard to come by," Colts general manager Ryan Grigson said in a statement. "He's a team first guy that works as hard at his craft as anyone I have ever been around regardless of position. His desire to be great literally consumes him and he is a shining example of the sacrifice it truly takes every single day to be at the top of his game come Sunday."
Castonzo's new deal will be for four years and $43.6 million, a source told ESPN.
The Colts' first-round pick in 2011, Castonzo has started all 60 games he has appeared in during his first four seasons. He was the only offensive lineman to start every game last season for the Colts.
Castonzo, who is responsible for protecting franchise quarterback Andrew Luck's blindside, played 1,400 snaps last season.
Colts owner Jim Irsay said during the offseason that his goal was to take care of extensions for receiver T.Y. Hilton and Castonzo before the start of the regular season. Hilton agreed to a five-year, $65 million contract extension last month.