The 31-year-old Lealiifano, who was diagnosed with leukaemia in 2016 but returned to rugby the following year after treatment, will depart after 12 seasons in the nation's capital.
"This was a difficult decision for me and I would like to thank the Brumbies for their support throughout my time at the club,” the captain and flyhalf said in a team release.
“The support that the Canberra public gave me and my family through a difficult time in my life will never be forgotten.
"I believe this club has a bright future under Dan (McKellar) and his coaching staff and look forward to following that progress."
Lealiifano, capped 19 times for the Wallabies, has played a big role in the Brumbies' resurgence this season as the Canberra-based team won the Australian conference and will play a home quarter-final against South Africa's Sharks on Saturday.
Lealiifano has previously played in Japan for Top League side Suntory and also had a stint with Irish side Ulster last year.
He is considered a chance to win a berth in Michael Cheika's Wallabies squad for the World Cup in Japan.
Lealiifano is the third high-profile player to announce his departure from the Brumbies in recent weeks, with Wallabies flanker David Pocock retiring from Super Rugby and winger Henry Speight having signed with the Queensland Reds.
(Reporting by Ian Ransom; Editing by Peter Rutherford)
