The leap in sales represents the highest growth figure since June 2008, according to Interactive Media Retail Group's latest e-retail sales index.
Shoppers spent £4.5bn online during the month, equivalent to £73 for every person in the UK.
Tina Spooner, director of information at IMRG, said: "With online sales growing at their highest rate in almost two years, this is good news for the e-retail industry. The fine weather in May, together with the build-up to the World Cup appears to have had a positive impact for e-retailers."
Alcohol sales leaped by 23% year on year, boosted by discounted price offers by many retailers.
Sales of electrical goods grew 14% year on year, while clothes sales rose 32% as shoppers responded to the warmer weather.