Castillo agreed to a five-year contract extension through 2014 valued at about $43 million. The deal will pay Castillo a guaranteed $17.5 million in signing and roster bonuses.
"You know I am not usually short of words but today is different," Castillo said. "It's an unbelievable feeling.
"Three years ago all I looked for was an opportunity, a chance to come into this league and play and prove the kind of player I was and prove the things I could do on that football field. And to be standing here three years later, committed to San Diego for seven more, it's an unbelievable feeling."
Castillo's contract, signed as a rookie in 2005, had been due to expire after the 2009 season. He said the Chargers approached him a few months ago about an extension.
"Everybody knows -- look around this team -- of the incredibly talented players we have," said Castillo, who missed six games each of the past two seasons with ankle injuries. "It's a credit to the Chargers to put themselves in this position. They have all these young guys who they are going to have to take care of. Look at their plan. They are doing everything they can do to keep us together.
Castillo has 13 sacks in 36 career games, including 33 starts. He had only 2½ sacks last year. Ankle surgery sidelined him for six games but he returned for the playoffs.
He was the second of the Chargers' two first-round draft picks in 2005 -- outside linebacker Shawne Merriman was the first -- and entered the league amid some controversy.
Castillo tested positive for androstenedione, which is considered a steroid by the NFL, at the 2005 scouting combine. He sent a letter to the 32 NFL teams alerting them of the situation, saying he took it after his final season at Northwestern because of a slow-healing elbow injury.
After being drafted by the Chargers, Castillo said it was "a huge mistake" to take the drug to help his performance at the combine. He promised to return his entire signing bonus if he ever tested positive again.
He's since stayed clean.
"Since his arrival, Luis has played a huge role in the success we've had as a defense," general manager A.J. Smith said in a statement. "He's an ascending player who is committed to getting better."
Agent Mike McCartney of Priority Sports in Chicago lauded the Chargers in working out "a great deal for both sides. It's going to be fun to watch Luis play at a Pro Bowl level the next seven years."
Coach Norv Turner said announcing Castillo's deal three days before the first full-squad workout of training camp "gives everybody a lift. It tells us what the organization is about. Obviously we are committed to keeping the guys that have been productive and Luis is an outstanding football player and it will make me sleep better each night. He's cranked up and ready to go. He has had a great offseason."
The Chargers fell one win short of going to the Super Bowl last season.
"Today is a great day but it is just the start of it," Castillo said. "There is so much I want to achieve on the field on an individual basis -- goals of my own -- but there is so much I want to help this team do, to be part of a Super Bowl in San Diego and to be here for a long, long time.
"As long as I can step on the field and look my teammates in the eye and know I have done my part, that is what I want to achieve."
Castillo will get a signing bonus of $6.15 million and roster bonuses of $4.35 million, $4.75 million and $2.25 million for three straight seasons.