The 21-year-old 6ft 5in Walmsley, who has earned rave reviews in his first full season with the Bulldogs, has agreed a three-year contract with Saints and will link up with them in the close season.
St Helens chairman Eamonn McManus said: "Alex is one of the most promising and exciting prospects in the game and we are delighted to have secured his services ahead of stiff competition."
He added: "He is a powerful forward who we have no doubt will suit Saints' style of play. We look forward to welcoming him into the club."
Saints will be hoping Walmsley can make a similar impact to Warrington's former Leigh prop Chris Hill, who made the 2012 Super League Dream Team just 12 months after leaving the Championship.