Love Rugby League exclusively revealed yesterday that Walker was set to join Saints.
Saints Head Coach Keiron Cunningham said: “Adam is a hard-working forward who will complement the dynamic pack we already have at the Saints.
“Along with his size, he has a big engine, good footwork and at 25-years-old it’s fair to say he hasn’t hit his peak yet.
“We’re looking forward to seeing him develop further at the club.”
Walker began his career at Huddersfield in 2010 before moving to Hull KR for the 2013 season.
There he played close to 90 games – and won the club’s Young Player of the Year award – as well as appearing 11 times for Scotland in both the World and European Cups.