The casual dining chain, which operates nine London restaurants and one in Selfridges Birmingham, will launch two further sites later this year in Peckham and Shoreditch.
YFM has taken a minority stake in the company and Sarah Willingham, formerly of PizzaExpress and Bombay Bicycyle Club, will chair the business.
Tonkotsu’s founders Emma Reynolds and Kem Yamada, who opened the first site in Soho in 2013, remain involved in the group.
Managing director Stephen Evans says Tonkotsu's financial performance has been “strong” over the past year despite "well documented issues" in the casual dining sector, and that the investment will allow the group to bring its ramen “to a greater audience”.
“This partnership will allow us to grow our business, enabling us to open a handful more Tonkotsus,” says Yamada.
“We chose YFM as our partner as we felt they shared our ambition to grow the business in the right way to maintain the culture and the brand.”
YFM also backed Leeds-based 'fresh fast food' restaurant group Friska with a £3m investment in 2017.
The Tonkotsu group previously ran high-end Asian concept Anzu in London’s St James’ Market development, but converted the site to its ramen brand before closing altogether in January after two years.