The deal is a three-year tie-up in which the event, owned by IMG Challenger World, will be named the Virgin Active London Triathlon. The value of the deal was not disclosed.
It amplifies the Virgin brand's association with sport that is being built by its five-year £17m sponsorship of the London Marathon, with Virgin Money as the lead brand, which started this year.
The deal will be supported by Virgin Active's own internal and external marketing activity, which will see all Triathlon communications branded with the health club's logo.
Virgin Active will help its members train and prepare for the event as part of its sponsorship by launching a range of classes especially tailored for the event alongside the clubs staging their own indoor triathlon next year.
Richard Baker, chairman of Virgin Active, said: "Virgin Active is very proud to be supporting the London Triathlon, given how well it fits with the core sports replicated in our clubs.
"This is the world's largest event of its type, and its reputation as the fastest growing Olympic sport allows us to encourage a wide range of people of all fitness levels, from beginner to elite, to get involved and try it for themselves.
"We are aiming to dispel the myth that you have to be an elite athlete to participate, and it's our aim to grow the triathlon sport through many of our club’s fun offerings, including a national indoor triathlon in 2011."
The triathlon will take place in London's Docklands area over the weekend of 30-31 July next year.
The event attracts more than 13,000 participants each year to its specially designed course that incorporates Big Ben, Tower Bridge and the London Eye.