The 24-year-old has signed a three-year deal with the Irish province and will move to Belfast after finishing the 2016 Super Rugby campaign with the Sharks.
Coetzee has 28 Springboks caps, having made his debut against England in 2012.
"It is a major coup for us to sign a player of Marcell's quality and experience," said Les Kiss, Ulster's director of rugby.
"His age, profile, positional versatility, leadership ability and character were some of the key ingredients as to why he was such an important target for us.
"He has played most of his international rugby between six and seven, but he has also played a huge amount at number eight, where we feel he could have a big impact for Ulster.
"Being appointed as the Sharks' vice-captain for the season shows that even at the young age of 24, Marcell has undoubted leadership ability.
"When you factor in the additions of Charles Piutau, Rodney Ah You and Kieran Treadwell, it will give us improved quality and depth across the squad in the coming seasons."
Current Ulster number eight, Nick Williams, is leaving to join Cardiff Blues at the end of the season.
South Africa back row Marcell Coetzee is to join Ulster in the summer.