The new variant was chosen by the public and received 19,000 of the total 35,000 votes cast.
It was chosen over violet, wild fruits and raspberry & pineapple, which were the other flavours put to the vote last month.
They had been shortlisted from 33,000 flavour suggestions from the drink's fans after brand-owner Global Brands asked people to suggest what the next VK flavour should be.
"Fans of VK have spoken in their thousands and they want Watermelon," said Jen Draper, head of marketing at Global Brands.
"Pink drinks are hot right now and our consumers seem to think so too, plus Watermelon is set to be a key flavour trend in 2019. However, it's not just about the latest on-trend colour – the taste is so important for VK and our in-house flavour development team has been working tirelessly to ensure the Watermelon flavour tastes as good as it looks!"
Watermelon VK will be launched in February 2019 and joins seven other flavours in the VK range: Blue, Ice, Strawberry & Lime, Orange & Passion Fruit, Black Cherry, Tropical Fruits and Apple & Mango.