The former Hull FC second-row, who joined from Huddersfield Giants at the end of ***, is closing in on 100 games for the East Yorkshire club.
Horne, 30, played with Rovers in the Challenge Cup final last month and has become an integral part of Chris Chester’s side.
“It was an easy decision for me; I’ve loved my time here and I’m looking forward to carrying that on,” he said.
“I’m really excited by where the club is going and this year was a big boost to the club in getting to Wembley
“I think I’ve had a really good season, probably one of the best I’ve had in a long time. I’ve been pleased with my consistency this year, I probably haven’t been a stand-out player but I’ve consistently done my job.
“As a team, we’ve shown in one-off games that we can beat anybody but we haven’t been consistent enough and injuries have taken their toll on us. I can definitely see that we are making progress though and hopefully we can improve again next season.”
Chester, who played alongside Horne at Hull FC, added: “Graeme is somebody who works hard for the team doing all the little things that his teammates appreciate and we’re really pleased to have him on board again next season.
“It’s important that we retain a lot of stability in the side next year to build on what we’ve achieved in 2015 and keeping hold of people like Graeme forms a big part of that.”
St Helens have confirmed the signing of France international half-back Theo Fages from Salford Red Devils on a four year deal starting in December.