Root, who will play in an overseas Twenty20 league for the first time, was snubbed by Indian Premier League teams this year and also dropped from the England side for a match against India in July.
Root had expressed fears he could be "left behind" in the format if he did not play it more. He has played just 28 T20 internationals in six years, with six of those games at the World T20 in 2016.
"I want to gain as much experience as possible in T20 cricket," Root said in a statement on the Sydney Thunder's website on Friday (www.sydneythunder.com.au)
"I'm sure there will be some mixed emotions for the fans (with me signing) but hopefully I can win them over through the time there.
"There's an opportunity for myself now to put in some strong performances and help Sydney Thunder go on and do some special things this year."
Root's England team mate Jos Buttler, who scored his first test century against India this week, has also returned to the side for the competition.
"It's brilliant for the club to get two players of their quality to join us," Thunder head coach Shane Bond said.
"Everybody knows Jos is one of the best T20 players in the world. They bring leadership to our team and you couldn't ask for two better players."
Root, 27, is not expected to be available for the entire tournament, with England's tour of Sri Lanka ending in late November before their tour of West Indies begins in January.
Australia's Big Bash runs from Dec. 19-Feb. 17.
(Reporting by Greg Stutchbury; Editing by Peter Rutherford)
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