Prior has been signed as a replacement overseas signing for Shahid Afridi, who is due to join up with Pakistan midway through the six-week tournament.
"Sydney Thunder has put together an excellent squad and I think we have a real chance this season," Prior told the club's official website.
"Also the Big Bash has fast become a competition that players all around the world want to play in, so I feel very fortunate to have this opportunity."
Prior will line up alongside star names such as Australia skipper Michael Clarke and also Chris Gayle - who like the 30-year-old is set to arrive late into the tournament, which begins on December 7.
Prior, who is set to fly to Australia immediately after England's Test series in India finishes on December 17, is looking forward to lining up alongside big-hitting West Indian Gayle.
"Definitely. Although standing at the other end could be dangerous at times," he said.
"I have played against him a lot in international cricket and there is no harder hitter of the cricket ball than him.
"I am as excited to play alongside him as I am sure the fans are to see him play."
The Thunder, who finished bottom of the inaugural BBL last season, also announced New Zealand opener Martin Guptill as an overseas replacement until Gayle's arrival.