The 34-year-old scored 35 goals in 115 appearances for the Toffees between 2008 and 2012.
"They have started this campaign well and that is a real boost for them," he told the club's official website.
"Everton is a great club and really enjoyed my time there. It will be special to come back and try to win.
"David Moyes is a very ambitious manager who wants to win and wants to improve. He wants the right spirit for his club and I think that has showed over the last few seasons."
Sunderland are currently 16th in the Premier League table with nine points from as many matches.