West Indies pair Shivnarine Chanderpaul and Marlon Samuels combined superbly to guide their side to a seven-wicket win in the rain-hit second ODI in Trinidad.
The win gave the home side a 2-0 winning lead in the three-game series.
Chanderpaul (52) and Samuels (54) hit six fours and five sixes between them as they made light of their revised target of 125 from 25 overs.
Sri Lanka had earlier reached 112-5 from 30.3 overs but rain twice interrupted their innings.
Opener Upul Tharanga's 40 from 82 balls was the best effort from the Sri Lankans with the bat, while Kumar Sangakkara made 28.
Sri Lanka started solidly but Mahela Udawatte was caught at backward point off Jerome Taylor in the ninth over.
Sangakkara joined Tharanga and they built on the work of the openers, but the West Indies' bowlers kept a tight rein on the scoring before Tharanga was caught at deep fine leg hooking at a short, rising delivery from Dwayne Bravo to leave Sri Lanka 78-2 in the 23rd over.
Rain soon stopped play with Sri Lanka on 83-2, but they resumed an hour later with a reduction in the number of overs to 47.
But the visitors struggled after that, losing Sangakkara, Kaushalya Weeraratne, and Chamara Silva for seven runs in the space of 13 balls before rain came down for a second time.
Nuwan Kulasekara boosted Sri Lanka's chances of winning when he dismissed West Indies opener Bravo, captain Chris Gayle and Ramnaresh Sarwan early on.
But the home side quickly gained control thanks to Chanderpaul and Samuels, with man-of-the-match Chanderpaul securing the win when he sent a delivery from Tillakeratne Dilshan into mid-wicket for a single.
The third and final ODI in the series will be contested on Tuesday at the Beausejour Stadium in St. Lucia.
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