The 30-year-old Australian-born forward represented Scotland at the 2013 Rugby League World Cup.
He holds the record for the most consecutive appearances in the NRL with 215 between 2006 and 2014.
"The club has a great profile and stature in the game and I hope to be able to contribute to developing that wonderful culture," he said.
Douglas will join St Helens ahead of the 2017 season.
"I am told Saints have the best fans in the game and I can't wait to engage with them. I'm really looking forward to this next chapter in my life," he added.
Super League side St Helens have signed prop Luke Douglas from NRL's Gold Coast Titans on a three-year deal.