The 23-year old defensive midfielder has been in the Scottish first division for the past four years, enjoying stints with Gretna, Stirling Albion and Morton.
Paartalu hails from Sydney and said he always imagined he would play professionally in Australia.
"My contract was running down with Morton in the summer and there was plenty of interest from Ange (Postecoglou)," Paartalu said.
"Working with him was a big draw to come back at the right time.
"I've been watching the progress of the A-League and I always imagined myself playing in it, it was just a matter of timing of when to come back.
"I feel the way things are going in Australia I'd like to give it a crack and with what Ange is trying to put together for next season it is going to be interesting and exciting.
"It was an opportunity I couldn't refuse really."
Paartalu has signed a three-year contract with the Roar.