The 23-year-old has penned an initial one year deal with the option of an extension at the Welsh region after negotiating his release from the Blue Bulls.
Following a succession of visa problems the former South Africa under-20s star is delighted to complete the move and is itching to get on the pitch.
"It's a big move for me, but it's something I've always wanted to do, go and play overseas," said Watermeyer.
"I'm very excited to join the Ospreys. It's been a while since we first spoke about coming over back in October, there were regulations to go through, but now I'm here I just want to get on the pitch again.
"There's a lot of good players here, with international exposure, who I'm looking forward to training with and trying to learn from, to understand the set-up.
"I'm here initially until the end of the season but I want to do enough to show the coaches that I can be an Ospreys player for a lot longer."
Watermeyer has not been registered for the Heineken Cup but Ospreys director of rugby is looking forward to seeing what the centre can bring to the Liberty Stadium.
Johnson also hopes he can become Welsh-qualified in three year's time if he remains at the Ospreys.
"He's going to bring typical South African resilience and grit in midfield, but there's a fair bit of skill in there as well," said Johnson.
"He's certainly an aggressive centre with a good pedigree in the game and I believe he can be a very good player not only for the Ospreys but, in the future, for Wales as well.
"We are allowed two non-capped foreigners in our squad so he joins Hanno [Dirksen] in our regulation two.
"He's young and he fits our profile. We didn't go to the market for a big name, we wanted someone who fitted our ethic.
"He has very good qualities that will I'm sure see him settling in very well at the region and having an impact."