The deal makes Aswin the first player capped by India to feature in the BBL. The overlords at the BCCI had prevented men from playing in global leagues around the world, but Ashwin's retirement from the Chennai Super Kings in the Indian Premier League has opened the door to other opportunities.
The India legend brings with him a wealth of experience, having claimed 527 scalps at Test level, won an ODI World Cup and featured in 333 T20 matches. He joins World Cup-winning coach Trevor Bayliss and captain David Warner.
"Thunder were crystal clear about how they'd use me and brave enough to back it," the 39-year-old told the Sydney Thunder website.
"My conversations with the leadership were excellent, and we're fully aligned on my role. I love how Dave Warner plays the game, and it's always better when your leader shares your mindset. I can't wait to perform for the Thunder Nation."
Trent Copeland, Sydney Thunder General Manager, was thrilled to have the experienced spinner on board.
“From the first time we spoke, Ashwin impressed everyone at the Thunder with his passion, desire to win and understanding of what makes our club special," he said.
"He will bring an injection of fresh energy and world-class bowling mid-tournament, while his presence as a leader and mentor will be invaluable for our young players.
"Perhaps most excitingly, Ashwin's connection with the rapidly growing Indian diaspora in Western Sydney will inspire a new wave of members and fans to join the Thunder Nation. We can't wait for them to become part of our journey."
However, the Thunder will have to wait for their new superstar. Ashwin has already signed up for next week's ILT20 auction and is the player with the highest base price. The Dubai tournament takes place from 2 December to 4 January, meaning he will be unavailable for the first three weeks of the BBL.