The 32-year-old left Queens Park Rangers in the summer - and was linked with a host of clubs.
And he has decided to leave English football for a new challenge in Australia.
"I'm delighted to be joining Brisbane Roar," said Austin.
"To play abroad is something I've always wanted to do and when the opportunity came up to join this club I was looking to explore it.
"I met the gaffer Warren in London and he sold me the project of the football club and it was one I was excited about!
"I want to help the club get back into the top six and on a personal note score as many goals as possible."
Austin signed for West Brom back in 2019 for a fee believed to be in the region of £4 million - and went on to score 10 goals in 34 league games as Albion were promotion to the Premier League.
He netted a handful of crucial late spot kicks in the season - but he would play just five times in the Premier League and left on loan in January.