Sri Lanka scored a challenging 302-9 after electing to bat against Bangladesh in the third and final one-day international in Pallekele on Thursday.
Opener Tillakaratne Dilshan top-scored for Sri Lanka with a solid 125, while Kusal Perera (56) and Kumar Sangakkara (48) were the other main scorers in the day-night match.
Bangladesh's left-arm spinner Abdur Razzak finished with 5-62 off 10 overs.
Sri Lanka lead 1-0 in the series following their eight-wicket win in the opening match in Hambantota. The second one-dayer at the same venue was abandoned due to rain.