Bangladesh needed eight runs in the last over bowled by Pathirana and Pawan Pathiraja produced a smart piece of fielding at mid-wicket to run out last man Ripon Mondol with a direct hit.
Earlier, Sri Lanka made 228 for eight from 50 overs after electing to bat first. Opener Sadeesh Jayawardena (58) and Pawan Pathiraja (51) made useful knocks to build up the innings.
Sri Lanka Under-19: 228/8 in 50 overs (Sadeesh Jayawardena 58, Shevon Daniel 34, Pawan Pathiraja 51, Raveen de Silva 25, Chamindu Wickramasinghe 27, Asikur Zaman 2/54, Ripon Mondol 3/49)
Bangladesh Under-19: 227 in 49.3 overs (Mahfijul Islam 75, Iftikher Hossain 36, S. M Meherob 33, Ariful Islam 23, Dunith Wellalage 3/30, Trevin Mathews 2/47, Matheesha Pathirana 3/26).