The Newport Gwent Dragons full-back will move from Rodney Parade at the end of this season, the Ospreys announced.
Evans, 25, won two caps on Wales' tour to North America in 2009.
"This is a great opportunity for me," he said on the club's official website.
"The Ospreys have been the most successful team in Wales over the last 10 years, winning leagues and producing players for the national team.
"There is some real world-class talent in the squad so it's a big challenge for me personally, but I can't wait to get involved and fight for my place."