Wellens, a former Manchester United trainee, told the club website: "This is a club with a big future that wants to get back in the Premier League.
"I'm delighted that I'll have the chance to be part of that."
Boss Sven-Goran Eriksson said the deal was "great" news, adding: "Richie is a fantastic player, a true professional.
"Technically, he is second-to-none. He is quick has great skill and vision and when he plays well, we play well.
"Richie is part of the heartbeat of the team and I believe that there is more to come from him in the future."
Wellens has made 90 appearances for City and missed just one league game during the 2010/11 campaign.
Source: BBC Sport
Source: BBC Sport