The home side, in pursuit of their 317-run target, were dealt an early blow when they lost opener Sami Aslam at the score of five.
Wickets kept on tumbling after that as Dilruwan Perera claimed three wickets to wreak havoc in the Pakistani middle-order.
Earlier, Pakistan dismissed Sri Lanka for a meagre 96 runs in the visitors' second innings courtesy some brilliant bowling Wahab Riaz and Haris Sohail.
Kusal Mendis was Sri Lanka's top-scorer with 29 off 49 balls.
Wahab was the chief destroyer for Pakistan with four wickets to his name, while Haris claimed three wickets in the 26th over of Islanders' second innings.
Earlier, Sri Lanka scored 482 in the first innings while Pakistan could only reply with 262.
The visitors lead the two-match series 1-0.
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