Cockspur Spiced will be sold only in 50cl bottles with an abv of 22% and an rrp of £10.
W&M said the launch was aimed at meeting growing demand for lower abv products and a trend towards fractional bottles in the off-trade.
UK sales director John Bradbury said: “We are increasingly seeing demand for convenience-sized offerings in the off-trade through the growth of 35cl and the decline of 1-litre bottles.
“Consumers want products which are accessible, available at a great price point and have a lower risk of purchase than 70cl.
“The size and abv also provides retailers with a great opportunity for strong profit margins.”
The launch is supported by in-store POS.
Cockspur parted company with previous UK agent Blavod Wines & Spirits in 2010 in a contractual dispute.
The brand – which W&M said was growing at 27% annually – is the official rum of the West Indies cricket team, currently touring England.
Cockspur is produced by Hanschell Inniss of Barbados.