Richards helped the Tigers claim their inaugural premiership in 2005 before leaving to join Wigan, where he has become record points scorer.
Richards, 31, who has also been Super League‘s top point scorer in four seasons, says he is delighted to be returning to the joint venture club.
“I have some incredible memories from my time previously at the club,’’ Richards told the Tigers’ official website.
“The premiership success in 2005 created a special bond amongst the players and with the supporters.
Former St Helens coach Mick Potter, who took over from Tim Sheens at Tigers said Richards would prove a “great asset” for the squad.
“Pat has been one of the best performing players in England for many seasons now. He was a winner of the 2010 Man of Steel award which recognises the season’s best player,’’ Potter said.
“His size, strength and skill as well as his experience and goal-kicking ability will be great assets for our squad. We look forward to welcoming Pat back to the Wests Tigers family.”