The 29-year-old has moved to the Kingspan Stadium until January 22, but his switch to the Pro14 is only brief as he will return to the Brumbies ahead of the new Super Rugby season.
Lealiifano, who was diagnosed with leukaemia 12 months ago before making a remarkable recovery to return to action in June, has revealed he is looking forward to the challenge of representing Ulster and he also thanked the Brumbies for sanctioning the move.
He said: "I'd like to thank the Brumbies for allowing me to do this stint abroad.
"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."
Ulster director of rugby Les Kiss believes Lealiifano will be a huge asset to his side and he has praised the character the Australian has shown to overcome his recent challenges.
Kiss said: "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."