Ahmed Shehzad hit 95 off 90 balls and Mohammad Hafeez contributed an 88-ball 70 to take Pakistan over the line in just 40.5 overs after its bowlers had restricted Sri Lanka to 256 for 9.
The result kept alive Pakistan’s hopes of qualifying for the eight-team ICC Champions Trophy 2017, to be played in England.
Sri Lanka is assured of taking part in the ICC Champions Trophy 2017, but was lacklustre in the crucial encounter at the R Premadasa Stadium.
Shehzad and Azhar Ali, the captain, put Pakistan on course for victory with an opening stand of 75 in 11 overs before Lasith Malinga removed Azhar for 33, scored off 28 balls.
Hafeez then helped Shehzad build on the start by adding 115 for the second wicket, but Sri Lanka hit back to remove both batsmen in the space of 23 runs.
Shehzad fell five runs short of his fourth ODI century when he cut Suranga Lakmal to Kusal Perera at third man after hitting 12 fours and a six in his innings.
Hafeez was then caught behind by Dinesh Chandimal off Milinda Siriwardana to leave Pakistan on 213 for 3, but Pakistan had reached a position of strength by then.
Shoaib Malik hit a 16-ball 29 and ended the match by lofting Siriwardana over mid-wicket for his fourth six, while Sarfraz Ahmed remained unbeaten on 17 off 23 balls.
