Business Growth Fund, an investment fund backed by the UK’s five largest banks – Barclays, HSBC, Lloyds, RBS and Standard Chartered – is providing the money in return for a minority stake in the gym chain.
HSBC is supplying a further £7m in fresh loans for the firm.
Clearwater International advised Gymbox on the deal.
Gymbox was founded in 2001 by former advertising executive Richard Hilton and now has five clubs across London.
The company expects to report revenue of £10m and core earnings of £3.5m in the next financial year.
“We look forward to opening new sites this year and next and introducing the Gymbox concept to a wider in audience in London and other UK cities,” said Hilton.