The 30-year-old prop has made more than 260 appearances in the NRL for Cronulla and the Titans and helped Scotland reach the quarter-finals of the 2013 World Cup.
Saints coach Keiron Cunningham said: "Luke is a dynamic, tough-running front-rower who will bring top level NRL experience to our pack next season.
"His leadership will be important to our younger players too and we are delighted to secure his signature for the next three years."
Douglas holds the record for the most consecutive appearances in the NRL after enjoying a run of 215 games from 2006 to 2014.
"I'm really looking forward to this new challenge," Douglas said. "The club has a great profile and stature in the game and I hope to be able to contribute to developing that wonderful culture.
"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."
Douglas is St Helens' second confirmed recruit for 2017 following the signing of Melbourne centre Ryan Morgan.
