The powerful ball carrying number eight will join from the Bulls.
Confirming the news Harlequins director of rugby John Kingston said: “Renaldo is one of the most destructive ball carriers in world rugby and so I am absolutely delighted to have secured his services on a long-term contract.
“As captain of his country, he also brings strong leadership qualities.
“He chose Harlequins over other options around the world and his signing is a major statement of where we want to take this great club over the coming years.”
A product of the Sharks Academy he has played 78 Currie Cup matches, most prominently for the Mpumalanga based Pumas.
In 2015 he made his Super Rugby debut for the Sharks and after spending the 2015 off-season with Japanese side Toyota Verblitz, he joined the Bulls in 2016.
Qualifying for Namibia through his mother, the 27-year-old made his international debut in 2014 against Kenya. Bothma played a key role in helping his country qualify for the 2015 Rugby World Cup, and made three appearances in the tournament against New Zealand, Tonga and Georgia.
He was appointed as captain of Namibia in June 2016 and has 13 international caps to his name, scoring 40 points.
Commenting on the announcement, Bothma added: “I’m really excited to move to England next summer and positively impact Harlequins. I’ve signed a long-term deal and I’m wanting to play a big role in helping grow the Club and bring it to even higher heights.
“I’m a big believer in a positive playing environment and from speaking to John Kingston, it is very clear that this is how Harlequins operates. The Club has a great vibe and there is a huge family friendly nature to it.
“I have another Super Rugby season with the Bulls to concentrate on and I will be giving it my all before my move to England.”
Source: Planet Rugby