Australia international Christian Lealiifano will join Ulster on a short-term deal before returning to the Brumbies in January.
The fly-half, who was diagnosed with leukaemia last year, made his Super Rugby return in July, having revealed earlier in 2017 that he was in remission.
Lealiifano will join up with the Pro14 side ahead of the 2017-18 season and stay until the Champions Cup group stages are over in late January.
He will then return to Australia in time for the 2018 Super Rugby campaign.
"I'd like to thank the Brumbies for allowing me to do this stint abroad," said Lealiifano, who has 19 caps for the Wallabies, the last of which came against England in June 2016.
"I'm really excited to be joining up with Ulster Rugby and I'm looking forward to getting out there in front of the supporters at Kingspan Stadium.
"I can't wait to experience the rugby and the culture in Belfast."
"This is a massive boost for us as we approach the start of the season," added Ulster director of rugby Les Kiss.
"It was paramount that we signed an experienced player to give us another option at fly-half and this is a fantastic solution for us.
"Christian has the playing talent, the leadership ability and the personality to make a big impact here. He has overcome significant challenges in his career and that is testament to his bravery and strong character."