He will move to Kingspan Stadium in the summer, following the conclusion of the 2016 Super Rugby season, during which he will represent the Sharks.
Coetzee, who attended Port Natal High School in Durban, made his Super Rugby debut midway through 2011 and he has already accrued 68 appearances for the Sharks in that competition.
The versatile forward, who can operate across the back row, has been named as captain of the franchise for their pre-season game against Toulouse this evening.
Coetzee also completed a season with Japanese Top League side Honda Heat, prior to his return to the Sharks. He scored three tries in nine appearances during that short stint.
Director of Rugby, Les Kiss, welcomed the news:
"It is a major coup for us to sign a player of Marcell's quality and experience. 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 6 and 7, but he has also played a huge amount at number 8, where we feel he could have a big impact for Ulster. That versatility is absolutely vital for us, as not only can Marcell be hugely effective across the three positions, but this can also accommodate the development of a number of our home-grown Irish-qualified players.
"Being appointed as the Sharks' vice-captain for the season shows that even at the young age of 24, Marcell has undoubted leadership ability. On the park, with his skilled and aggressive approach, he clearly leads from the front.
"Bryn (Cunningham) has done a marvellous job in securing the services of such a talented player and I look forward to working with him. 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."
Operations Director, Bryn Cunningham, added:
"To attract someone of Marcell's stature is a real statement of intent on our behalf. We have very high aspirations of what we want to achieve over the next two to three years and as a result, we are trying to build a squad that is capable of being extremely competitive in both the Guinness PRO12 and in Europe.
"At 24, we believe his best years are still ahead of him, which is hugely exciting and shows the commitment Marcell has given to Ulster Rugby. I know that this was an incredibly difficult decision for him to make with the attachment he has to the Sharks, not to mention the high level of interest from other clubs.
"Having spent plenty of time talking with Marcell, it is also evident that he has all the key character attributes, like many South Africans that have gone before him at Ulster, to fit into our culture seamlessly and have a massively positive impact on our environment.
"We are all very excited about what he will bring to Ulster in the coming seasons and we wish him success this season with the Sharks."
Guinness PRO12 Ulster News
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