Steven Kitshoff will join Ulster after the World Cup.
He was part of the team that won the World Cup in 2019.
He was the captain of the Stormers last season.
"It's clear that Ulster is on an exciting journey, and it's one that I would love to be a part of," he said.
I want to be part of the squad that takes that next step and secures silverware for the province.
Next year I will be moving to Belfast.
"It's going to be a change, but I know, from everything I've heard, that when I line out for the first time in front of a home crowd, it's going to be a really special moment."
The signing of a British and Irish Lion is a sign of strength.
In September and October, France will host the World Cup.