Lealiifano on Monday night announced he would link up with the Belfast-based club for the next five months before returning to pre-season training with the Brumbies in mid-January.
The deal is designed to ensure the playmaker gets more game time after he missed nearly all of the recently-completed Super Rugby season while recovering from his illness.
"I feel that it's important that I get some minutes playing competitive rugby ahead of the 2018 Super Rugby season," Lealiifano said in a statement.
"With this opportunity, I feel I can come back to the Brumbies in great shape."
Lealiifano underwent chemotherapy and had a life-saving bone marrow transplant in late 2016, and in February was told by doctors that his cancer was in remission.
The 29-year-old made his top-level return in the Brumbies' quarterfinal loss to the Hurricanes last month, coming off the bench in the second half.
"We see a real benefit for Christian in joining Ulster for a brief period both on and off the field," Brumbies coach Dan McKellar said.
"On field, it is about Christian getting an opportunity to play professional rugby for a good club in a very strong competition.
"Off field, it is about Christian and his family getting the chance to experience a new culture and enjoy new life experiences that he was unfortunately denied 12 months ago."
Ulster start their Pro14 season on September 1 against the Cheetahs, one of the two South African sides cut from Super Rugby at the end of last season.
Lealiifano has played 19 tests for the Wallabies, with his last appearance coming against England last year.
Photo by: MARK NOLAN/GETTY IMAGES Caption: After missing most of the 2017 Super Rugby season, Christian Lealiifano will head to Ulster for five months before rejoining the Brumbies in January.