The powerful ball-carrying number eight, 27, who has been capped 13 times by his country, will move from South African Super Rugby side the Bulls.
He made three appearances at the 2015 Rugby World Cup, including one against eventual winners New Zealand.
"I'm really excited to move to England next summer and positively impact Harlequins," Bothma said. "I'm wanting to play a big role."
Bothma has also previously played Super Rugby with the Sharks while in-between his two spells in South Africa he appeared in Japan with Toyota Verblitz.
Namibia captain Renaldo Bothma will join Harlequins from next season.