Defender Robert Cornthwaite has signed a new three-year contract that will keep him at Adelaide United until 2013.

The 24-year-old, who was born in England but raised in Adelaide, made his Australia debut this season and the new deal will see him become one of the club's longest-serving players having been with Reds since the A-League's inception in 2005.

"I'm an Adelaide boy so there's no other team I want to play for, and I'm really happy to be staying at Adelaide United," Cornthwaite said.

"As soon as I heard that the club wanted to re-sign me I was really happy and we went through the negotiations, which went really smoothly, so I'm very happy to be staying here.

"Obviously Viddie (boss Aurelio Vidmar) and the club have shown a lot of faith in me giving me a three-year deal. I think over the five seasons that I've been here I have proven that I have improved each year and have been willing to learn and become a better player.

"I'm always going to make mistakes but as long as I limit those and learn from them, in three years time hopefully I'll be a very strong leader at the club."

Cornthwaite, who was suspended by the club earlier this season following an altercation at an Adelaide nightclub, is one of only three surviving players from the original 2005 squad along with skipper Travis Dodd and midfielder Lucas Pantelis.

"It's a bit of an honour for me to still be here as one of only three players still here from the first season, and by the time this contract expires, I would have been here for eight years so it's a pretty big honour," he added.

"Not a lot of players in the A-League have stuck around at one club as long as me so I'm really proud of my achievement. To play at one club for your whole career would be something special and this is a club that I love and is close to my heart, so it would be an honour to stay here for as long as possible.

"I'm still relatively young and inexperienced for a centre half so my best years are definitely still to come, there is still a lot of improvement I need to make in my game.

"The club has had a lot of success without winning a championship so that's something all of the players at the club are striving towards."