The 22-year-old was only signed on a one-year deal by the Broncos for 2021, and never came into calculations for the first-grade squad despite a strong Queensland Cup season with the Souths Logan Magpies.
The team weren't able to win the competition, but Tass scored nine tries in 15 games for the Magpies on their way to the second week of the finals in Queensland's top state-based competition.
Tass also kicked 45 goals, and has proven himself to be more than capable with the boot, which is an area South Sydney will need to sort out for 2022 following the departure of long-term halfback and goal-kicker Adam Reynolds.
It's thought Blake Taaffe may take over the kicking duties from Reynolds, but their depth in that department looks a little shaky.
Rabbitohs head of football Mark Ellison said Tass would have the opportunity to debut in 2022.
“We’ve been very impressed with Isaiah in our discussions to bring him to our Club and we’re very happy to have him joining us this pre-season,” Ellison said.
“He has been part of full-time training setups before and knows he will need to work hard to give himself the opportunity to make his NRL debut at our Club in 2022.
He has great skills, is very athletic and has the backing of other players at our Club who have played with him or seen him play.
“We’re looking forward to having Isaiah join us for 2022.”