Tremain will join England's big-hitting batsman Alex Hales, South African all-rounder Chris Morris and former Brisbane Heat middle-order batsman Alex Ross in the new Thunder squad.
“I don't think my game has changed too much. I think I've changed quite a bit. I've played a lot more cricket; experienced a lot more. I've had a lot more success, and I've failed a lot more. I've had a career, basically,” said Tremain of the years since he last represented Thunder.
The right-hand pacer had previously played for Thunder between 2012 and 2015. He played the first of his four matches for Australia in 2016 in the ODI series against South Africa.
Sydney Thunder will open their BBL 9 campaign against the Brisbane Heat at the Gabba on December 17.