The 23-year-old centre has signed on an initial short term deal until June next year to strengthen Scott Johnson's back division in their battle for the RaboDirect PRO12 title.
And having lost the versatile James Hook in the summer to French side Perpignan Watermeyer, who played for South Africa U20s, is hoping to make an impact sooner rather than later.
"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," he said.
"It's a big move for me, but it's something I've always wanted to do, go and play overseas. It's going to be a great experience for me, something of the unknown for me, but that's what I like.
"It's a personal challenge for me, taking me out of my comfort zone. I'm excited and looking forward to showing what I can do here.
"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."
And Johnson, whose side currently sit second in the RaboDirect PRO12 table, is hoping Watermeyer can be the next in a long line of successful non-Welsh players to make an impact at the Liberty Stadium.
"Just as we develop local players, we've got a proven track record of identifying non-Welsh players and through our sound development principles are able to develop them so they reach their full potential," added Johnson.
"People like Tommy Bowe and Nikki Walker, as well as someone like Filo [Tiatia] who we were able to reinvigorate and give a new lease of life to," Johnson said.
"The difference with Stefan is he could become Welsh qualified meaning the whole country would benefit."
63x46.png){kind=logo}