Robinhood is an app built around one promise: no-fee stock and cryptocurrency trading.

A little over four years after launch, the company has a $US5.6 billion valuation – four times more than its 2017 valuation of $US1.3 billion.

The app first launched in December 2014 and quickly became a favourite among younger people looking to invest.

It allows users the freedom to complete a transaction without paying a processing fee, and became the first finance app to win an Apple Design Award thanks to its simple-but-stylish design.

While Robinhood has not yet launched locally, in 2015 it announced Australia would be its first international market.

Aussie millennials are already fans of crypto trading and US tech stocks.

The founder of an app that allows Australians to cheaply and easily buy US stocks told Stockhead recently that millennials want to invest in the names they know – Spotify, Facebook and the like – rather than ASX stalwarts like banks or resources.