Roby, 25, has been handed the iconic number 9 shirt for 2011 following the retirement of Keiron Cunningham, and will now stay at the club until at least the end of 2014.
Roby stated: “It was a big but ultimately easy decision for me. The Saints have been my only club and are only going to get stronger.
“We are moving to a fantastic new training facility this season and to a new stadium the next. We’ve got a top new coach in Royce Simmons and have made some great new signings for next season.
“The next few years will be more than exciting for the Club and at a time when I should be reaching my own peak.”
The England international has racked up 203 appearances for Saints, scoring 58 tries in the process.
Saints Chairman, Eamonn McManus, added: “This is a big boost for us and a clear endorsement of the future of the Club.
“James is one of the best players in the world and crucial to our continued success. Having come through our academy and been at the heart of our success in recent years, he represents all that we strive for and produce at the Club.”