Shadab joins the second-placed Sixers as an overseas player, and his presence looms as a vital cog following a season-ending neck stress fracture for off spinner Ben Manenti and short-term calf strain to veteran tweaker Steve O'Keefe.
The 23-year-old Shadab made his debut for Pakistan in all three formats while still a teenager and now has 64 T20 international matches under his belt, having taken 73 wickets at an economy rate of 7.11.
The star leg spinner averages 18.33 with the bat at a strike rate of 136.81.
Shadab has also played 48 ODIs and six Tests.
At the recent T20 World Cup, Shadab played in all six matches for Pakistan and finished the tournament with nine wickets.
He snared 4-26 in the five-wicket semi-final loss to Australia, taking the prized wickets of Dave Warner, Mitch Marsh, Steve Smith, and Glenn Maxwell.
"Shadab joins us as cover for the bowlers we have lost and comes together with two other young and exciting spinners in Lloyd Pope and Todd Murphy," Sixers coach Greg Shipperd said.
"All three young men are still learning their craft but have shown at various levels that they can be world class.
"We welcome Shadab's skills in all three facets of the game and can't wait to see him get an opportunity in coming matches."
The Sixers (4-1) will be back in action on Boxing Day when they face the Sydney Thunder at Sydney Showground Stadium.
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