South African No.8 Duane Vermeulen will arrive in Belfast in November and will most likely square up to Saints in the European encounters in December and January.
It means he could be involved in a titanic tussle with another South African No.8, Juarno Augustus, who arrived in Northampton this summer.
Vermeulen's arrival will add another dimension to a tough Champions Cup fixture list for Saints, who must also face French powerhouse Racing 92 twice in the pool stages.
And Ulster are delighted to have recruited the 35-year-old, who stands at 6ft 4in and is nicknamed the Thor of rugby.
“Duane is clearly a world class player who, as the current world champions’ starting eight and MVP from the last RWC final, is set to bring depth and experience to our really talented group of back rowers,” said Ulster head coach Dan McFarland.
“As well as being an undeniable talent on the pitch, I know Duane will play an important role going forward in strengthening the winning mindset we are building as a squad.
“I would also like to take this opportunity to recognise Bryn Cunningham’s brilliant work in orchestrating this deal from start to finish.
"It was always going to be a difficult ask to find a difference-making eight, but Bryn has definitely achieved that.”
World Cup winner Vermeulen has signed a two-year deal at Ulster.
And the player said: "Ulster and myself had detailed discussions about the club’s values, expectations and the potential role I can play going forward.
“It appealed and resonated with me to a point where I got very excited and decided to commit myself to this Ulster journey.
"I cannot wait to become part of the Ulster family and to serve as best I can.”