"This represents a tremendous signing for Sunderland," said the Black Cats boss.
"Kelvin has played in the Premier League before with Wimbledon and he has played something near 40 games a season for the last four years.
"He was voted the best goalkeeper in the Championship last season and at 28 he is also in his prime age for a goalkeeper.
"He has terrific ability and is experienced, so we're delighted to have Kelvin in our squad for the Premiership."
Davis becomes the clubs forth signing of the summer following the recent captures of strikers Jonathan Stead and Daryl Murphy and defender Nyron Nosworthy.