Darren Bravo top scored with 54 while Dwayne Bravo (43) and Kieron Pollard (30) came up with handy cameos to post 232 for 8 in 50 overs, while Pakistan folded for 195 off 47.5 overs in reply with Sunil Narine returning figures of 4 for 26.
Pakistan picked up the wicket of Chris Gayle in the first over when Mohammad Irfan had him caught at midwicket. Johnson Charles and Darren Bravo then steadied the innings with a 79-run stand, but once Pakistan broke through, West Indies found the going tough on a tricky pitch that appeared to get slower as the game progressed.
But Pollard and Darren Bravo, who got together at 150 for 5 in the 39th over, took the innings from its state of uncertainty to 220 before Pollard was dismissed by Asad Ali in the penultimate over.
In its chase, Pakistan appeared to be cruising at 103 for 2 with Misbah-ul-Haq well entrenched and Nasir Jamshed (54) riding his considerable luck towards a half-century. But Misbah played a shot totally out of character to be bowled by Darren Sammy, a fitting reward for the all-rounder playing his 100th ODI.
Narine then ran through the lower order to claim the Man of the match award. Umar Akmal provided some spark with a fighting half-century, but that was scant consolation for Pakistan.
With the series now level, the teams travel to St Lucia for the remaining three matches, beginning on Friday, before the short tour concludes with two Twenty20 Internationals in St Vincent on July 27 and 28.