Inflexion will work with the retailer's existing management team, providing capital and support for the next stage of growth, in particular expansion into Japan and the US.
Inflexion managing partner Simon Turner said: 'Jack Wills operates in an interesting niche market and has developed strongly through an excellent understanding of its consumer demographics.'
He added: 'We are very excited to be joining the experienced management team as a financial partner during this key stage of the company's development and to be working with such enthusiastic entrepreneurs to further enhance growth.'
Turner led the transaction on behalf of Inflexion and will join the retailer's board.
Jack Wills operates 20 stores.