The powerful 27-year-old South African tighthead prop joins the province following a season in the French Pro D2 with USON Nevers.
He has previously played for French Nationale side, CS Bourgoin-Jallieu, after spending over six years with Munster Rugby.
Knox attended the renowned Michaelhouse School and was offered a place in the Munster Rugby Academy at 18 years old, after impressing for his province, KwaZulu-Natal, at age-grade level.
Knox featured eight times in the season Munster won the URC title back in 2023. The front-rower made a total of 33 appearances for Munster before pursuing opportunities in French Rugby.
Irish-Qualified through residency, the forward returns to Irish Rugby with an ambition to progress his game in Ulster.
Commenting on the news, Knox said:
“I am delighted to be joining Ulster Rugby this summer, and I can’t wait to meet the staff and my new teammates in pre-season.
“Since leaving Ireland, I have gained more playing experience in France and feel I am coming back a more well-rounded player.
“I am looking forward to joining a very talented team and developing myself by working alongside international players. See you all soon!”
Ulster Rugby’s General Manager, Rory Best, added:
“We are very pleased to be bringing Keynan back into Irish Rugby and have no doubt he can make an impact for us going forward.
“He has an excellent physical profile and is strong around the set piece.
“With Clarke Dermody coming in as our new Forwards Coach, we believe that Keynan will really benefit from working alongside him and competing with our talented forwards.
“At 27 years of age, Keynan has the best years of his career ahead of him and we are excited to see what he can bring.”