The 31-year-old, who has been the top points scorer in the Super League four times, will return to his native Australia to join his former club West Tigers.
"This has been a difficult decision to make as I have thoroughly enjoyed my eight seasons with Wigan but the time has come for my family and I to return home," Richards told the club's official website.
"I will take with me many happy memories and I can only thank everyone for making me so welcome. I have been extremely fortunate to achieve the successes I have with Wigan with the team and individually and am hoping I can end my time here in England with more silverware to come this season."
Chairman Ian Lenagan added: "We are obviously disappointed to be losing Pat but fully respect his decision and his reasons to head home. Pat joined us in 2006 and he has been a wonderful player for the Wigan club and he has provided us all with some terrific moments. He is a model professional, is highly respected by everyone and is one of the best overseas signings the club has ever made."
Richards is close to racking up 2,300 points for the club following his arrival in 2006.