The 26-year-old is expected to play five games for the Sixers before joining up with England's limited-overs squad in India.
"To get the opportunity to play at the SCG is obviously a massive ambition of mine," he told the club website.
Roy could make his debut in the game against Sydney Thunder on 20 December.
He played three Big Bash games for Thunder two seasons ago.
"Jason is known as an explosive weapon at the top of the order and we look forward to his contribution, bringing also his English experiences from which we can share," said Sixers head coach Greg Shipperd.
England begin the limited-overs section of their tour to India next month.
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Surrey and England batsman Jason Roy has agreed a short-term deal to play for Sydney Sixers in the forthcoming Big Bash tournament in Australia.