Fletcher agreed a four-year contract after Wolves accepted an improved offer from Black Cats boss Martin O'Neill.
The 25-year-old had been on O'Neill's wanted list for much of the close-season and some reports claim he has paid around ?15 million ($23 million) to finally get his man.
"It has been no secret that we needed to strengthen our attacking options and Steven has a proven track record of scoring goals," O'Neill told Sunderland's website.
Even though Wolves were relegated from the Premier League last season, Fletcher still impressed and got on the scoresheet 12 times.
He scored the same amount the season before, to make it 24 goals for the club in 45 starts.
That prolific form made Fletcher the top target for O'Neill as he looked to bolster his attack following the departures of Nicklas Bendtner and Asamoah Gyan.
"I'm delighted. I've just come from the training ground and I'm delighted it's all gone through," O'Neill added.
"I think Steven will be terrific for us. He's underrated, but not by me.
"I think he's capable of scoring goals and linking play which is very important to us and I think now at 25 years of age he's got room to improve and really get better.
"Wolves lost their Premier League status last season but I think he's very good and I think he's improving."
Fletcher's transfer was registered in time for the former Burnley player to make his Sunderland debut in Saturday's clash against Reading, but O'Neill admitted he hadn't decided whether to pick him yet.
"He hasn't played that much. He had a setback a few weeks ago so in terms of fitness he didn't train with us today but I'll assess him anyway," O'Neill added.
Source: AFP
Source: AFP