Pakistan, who had won the first match, looked to be on their way after opener Azhar Ali made 96. However, his departure signaled a late order collapse as Pakistan lost its last five wickets for mere 47 runs.
Earlier, Sri Lanka scored a challenging 280-4 after electing to bat against Pakistan in the second one-day international in Pallekele on Saturday.
Opener Tillakaratne Dilshan top-scored for Sri Lanka with an unbeaten 119, hitting one six and 11 fours in his 139-ball knock.
Pakistan lead 1-0 in the five-match series following their six-wicket win in the opening one-dayer on Thursday.
Pakistan made one change as they brought in debutant paceman Rahat Ali in place of injured Mohammad Sami, while Sri Lanka retained the side.
Sri Lanka: Mahela Jayawardene (capt), Tillakaratne Dilshan, Upul Tharanga, Kumar Sangakkara, Dinesh Chandimal, Angelo Mathews, Lahiru Thirimanne, Thisara Perera, Nuwan Kulasekara, Lasith Malinga, Rangana Herath.
Pakistan: Misbah-ul-Haq (capt), Mohammad Hafeez, Azhar Ali, Younis Khan, Umar Akmal, Shahid Afridi, Sarfraz Ahmed, Umar Gul, Saeed Ajmal, Sohail Tanvir, Rahat Ali.
Umpires: Ruchira Palliyaguruge (SRI) and Paul Reiffel (AUS)
TV umpire: Ranmore Martinesz (SRI)
Match referee: Chris Broad (ENG)
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