Roby has made 518 appearances since his debut against Widnes in March 2004.
“I was aware that last year I said this was going to be my last and I was fully preparing for that,” he said.
“The more the season has gone on, I was thinking otherwise.
“I have thought about all the pros and cons and I have had a lot of conversations with my family and my wife and we have got to the point where I am feeling good and confident with my decision.”
Roby has scored 116 tries for Saints during his career and has helped the club win numerous trophies.
He has won the Man of Steel award as well as four Challenge Cups, the World Club Challenge, five Super League titles, along with 31 England caps.
“This is the biggest re-signing the club has made this year,” boss Kristian Woolf said.
“We have all seen the level of footy he has been playing. He can still play long minutes every week and still has a real impact on the game with his skill set, his defence and of course his leadership.”