The company, which offers its signature flat iron steak for just £10, will use the funding to continue to build its team and open additional sites.
Flat Iron founder Charlie Carroll will remain as managing director and retain a controlling stake in the business.
It is the latest investment for Piper, which also backs Be At One, Turtle Bay and Hickory’s. Global investment bank GCA Altium advised on the deal.
Carroll said Piper shared his vision to build restaurants that focused on being ‘better and different at every stage of the guest experience’.
Flat Iron launched in 2012 as a pop-up above a London pub, and has since grown to four sites in Soho, Covent Garden, Tottenham Court Road and Shoreditch.
A further two restaurants in Golborne Road and King’s Cross are set to open later this year.
The company won awards for Most Effective Restaurant Design and Best Value Restaurant Operator with fewer than 20 sites at Restaurant Magazine’s R200 Awards last year.