SoftBank's move comes less than three months after it first acquired shares in Banco Inter as part of an offering that raised 1.3 billion reais ($318.49 million).

Banco Inter also said its controlling shareholders, Rubens Menin Teixeira de Souza and Joao Vitor N. Menin, had reached a shareholder agreement with SoftBank, allowing it to appoint a board member.

Banco Inter, which listed its shares less than two years ago, has been growing at a breakneck pace, with the number of its clients surging to 2.5 million in June, up more than 250% from a year earlier.

SoftBank is making a play in Brazil's banking sector with the proceeds from its $5 billion Latin America fund, raised in March. In Brazil, the top five banks hold 82% of total assets.

Besides Banco Inter, the Japanese conglomerate also invested in the lending platform Creditas, which offers loans backed by assets like homes and autos, a market nearly untouched by big banks.

It is not just Banco Inter's client base that has accelerated at a rapid pace - its preferred shares are up roughly 200% this year, compared with less than 10% for shares in Brazil's largest listed lenders.

Banco Inter's market capitalisation is 13.6 billion reais.

(Reporting by Carolina Mandl; Editing by Edmund Blair and Paul Simao)

By Carolina Mandl