The 24-year old made his NRL debut in 2015 and has appeared in 17 first-grade games for the Bunnies.
Kennar was thrilled to secure a deal with Anthony Seibold’s team, who coached him at the Rabbitohs.
“I am very excited to have gained this opportunity,” he told the club website.
“I have really enjoyed the pre-season since I arrived here in Brisbane and have tried to impress at training and in the trial matches.
“Now I have a spot in the squad here I will do my very best to try and make the Broncos’ NRL team.”