Harper, 40, who has been without a club since quitting Championship team Hull City at the end of last season, has agreed a short-term deal that will keep him at the Stadium of Light until the end of the campaign.
"I turned down a lot of offers in the summer," he said. "I had a chance to join a Premier League club and a Championship club and had three offers to go to India but I think my career proves I'm a bit of a home bird."
Harper said he was surprised to receive a phone call on Thursday from Sunderland manager Sam Allardyce, who had a spell in charge at Newcastle from 2007-08.
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"I was in shock when he rang but over the course of today, seeing the training field, and that little gnawing feeling that there was something left in me, I thought 'Why not?'," the keeper added.
Second-bottom Sunderland host fellow relegation strugglers Bournemouth on Saturday.
(Writing by Tony Jimenez; Editing by Ken Ferris)
