The 25-year-old New Zealand international has made 43 appearances for Saints since joining them from Sydney Roosters at the start of the 2010 season.
"We are delighted to lock down Sia to a long-term deal," St Helens chairman Eamonn McManus said. "He has been one of our stand-out performers this season and at 25 years of age is still improving.
"He is a proven international and, alongside some of the fantastic products from our famed Academy, will form the hub of the side for many years to come."
Soliola added: "There was never any doubt that I would sign for the Saints. I have enjoyed my time at the club and I am looking forward to a successful season in our brand new stadium.
"We have a lot of strength in depth at Saints and I'm sure we will be challenging on all fronts this year."