Electing to bat, Kolkata scored a very competitive 161 for three on a tough pitch, thanks to a consistent Jacques Kallis and a marauding Yusuf Pathan, who sent the Somerset bowlers on a leather-hunt during the last six overs.
Kallis batted through the innings for his unbeaten 74 and Pathan hit a 21-ball 39 as the duo shared 90 runs for the unconquered fourth wicket.
The British team responded with tremendous power-hitting through Merwe and chased down the target with two balls to spare at Rajiv Gandhi International stadium.
As Merwe came out firing on all cylinders, Somerset were comfortably placed at 119 for one in 13 overs but they made it trifle difficult for themselves by losing three quick wickets before sealing the win.
Merwe's powerful knock was adorned with nine fours and two sixes and he shared 115-run stand with Peter Trego (28).
Somerset had lost the opener Chris Jones (6) to first ball of Iqbal Abdulla's spell but they still raced to 50 inside fifth over with Merwe scoring the bulk of the runs.
South African Merwe took the Kolkata attack by the scruff of its neck and was hitting all the bowlers cleanly and effortlessly. Such sweet was his timing that even his reversesweep off Shakib Al Hasan sailed over the boundary easily.
Somerset collected 63 runs in the first six overs, laying the platform for an easy victory.