In doing so, the 29-year-old will become the fifth Welsh star to leave the country within the last year, as a frugal wage structure and uncertainty over the future of the European competition threatens the future of the Pro12 division.
"While the future for Welsh rugby remains shrouded in uncertainty, situations like this remain, unfortunately, inevitable," read a statement on the club's website.
"But as a business we will continue to strive to make the right decisions for the long-term future of Ospreys Rugby."
Andy Lloyd, the club's operations manager, thanked the South African-born player for the eight years he spent at the Liberty Stadium.
"On behalf of the Ospreys, I'd like to thank Ian for his years of service to the region," he said. "It's not the way that either he or the Ospreys would have wanted to announce his departure.
"Unfortunately, due to there being so many unknown variables in the Welsh rugby landscape, incidents like this will continue to happen at every region."
George North was the other player to depart Wales, leaving Scarlets to link up with Northampton last summer.