Umar Akmal scored 77 runs and Misbah-ul-Haq 72 while Suranga Lakmal took two wickets as Pakistan overhauled the victory target in 39.5 overs for the loss of 4 wickets.
Earlier, seven wickets between Pakistan's Aizaz Cheema and Saeed Ajmal helped to bowl Sri Lanka out for 188.
Kumar Sangakkara (71) and Upul Tharanga (56) added 96 for the fifth wicket, putting Sri Lanka on course for 280, but with the demise of the latter, they succumbed and proceeded to lose their last six wickets for 27 runs. Cheema finished with 4-43 and Ajmal took 3-27.