Chasing a target of 306, Sri Lanka were bowled out for 268 in 48.1 overs, giving the hosts a winning start to the tournament, which also features England.
After winning the toss, Sri Lankan captain Kusal Mendis elected to field first. Pakistan were dismissed for 305 in 49.5 overs, with Salman Ali Agha scoring a century.
Pakistan lost both openers early, with Saim Ayub departing for seven and Fakhar Zaman scoring 16. Babar Azam and ODI debutant Saad Baig then steadied the innings with a 74-run partnership for the third wicket.
Babar scored 45 off 52 balls, including six boundaries, before being dismissed. Saad Baig contributed 39 runs before Pakistan lost their fourth wicket at 118.
Salman Ali Agha and Hasan Nawaz rebuilt the innings with a 97-run partnership for the fifth wicket. Hasan made 37 before departing, while Arafat Minhas added 18 in a 64-run stand with Salman.
Salman went on to complete the fourth century of his career, providing the backbone of Pakistan's innings before being dismissed at 291. Shaheen Shah Afridi scored 13 and Faheem Ashraf made six in the closing stages, while Naseem Shah and Haris Rauf were dismissed without scoring.
For Sri Lanka, Eshan Malinga claimed four wickets, Asitha Fernando took three and Dilshan Madushanka picked up two.
Sri Lanka showed resistance during the chase but could not reach the target, eventually being bowled out for 268 in 48.1 overs as Pakistan secured victory by 37 runs.
Team News and Series Schedule
The match marked the ODI debut of Pakistan wicketkeeper-batter Saad Baig. Pakistan captain Shaheen Shah Afridi had said at the toss that the pitch looked fresh and that he would also have chosen to bat first. He added that the team aimed to post a total of around 300.
Kusal Mendis said Sri Lanka had performed well in the final match of their recent T20 series against Pakistan but acknowledged that the ODI format presented a different challenge. He said Sri Lanka had selected three fast bowlers and one spinner.
According to the Pakistan Cricket Board, leg-spinners Shadab Khan and Sufiyan Muqeem missed the opening match due to illness. The team's medical staff is monitoring both players, with decisions on their availability for upcoming matches to be made later.
Pakistan entered the ODI tournament after winning the preceding three-match T20 series against Sri Lanka 2-1. Pakistan won the first two matches, while Sri Lanka claimed victory in the final game.
The tri-nation ODI series will be played in Rawalpindi and Lahore. Pakistan will face England in Rawalpindi on October 20, followed by England's match against Sri Lanka at the same venue on October 22.
The remaining group-stage matches will take place at Lahore's Gaddafi Stadium. Pakistan will play England on October 25 and Sri Lanka on October 27, while England and Sri Lanka will meet on October 29.
Each team will face the other two sides twice in the opening stage. The top two teams in the points table will then meet in the final in Lahore on October 31.
