The goalkeeper also turned down offers from the Championship before joining the Black Cats.
The goalkeeper had been out of contract since the end of last season, after his deal with Hull City came to an end and was not renewed.
He has been brought in as back up to duo Vito Mannone and Jordan Pickford after Costel Pantilimon was allowed to leave Watford earlier this week.
A move to the Stadium of Light nearly didn’t happen though, with Harper revealing that he had turned down offers from Premier League and Championship clubs, as well as several in India.
“I turned down a lot of offers in the summer,” he told safc.com.
“I had a chance to join Premier League club and Championship club in August and three offers to go to India, but I think my career proves I’m a bit of a home bird.”
“I was in shock when he [Sam Allardyce] 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.”
Harper will know that he is unlikely to find much game time at the Stadium of Light between now and the end of the season, unless injuries give him his chance.
Indeed, the first team spot now seems to be youngster Jordan Pickford’s to lose, with the youngster having come in and done well in recent weeks.
He has impressed, despite conceding seven in his last two games, and with many rating him, including Sam Allardyce, he looks set to kick start his Black Cats career in the second half of this campaign.