Stock trading app Robinhood is becoming a cryptocurrency broker.

Customers in California, Massachusetts, Missouri, Montana and New Hampshire will be able to trade bitcoin and ethereum through the app beginning in February, Robinhood announced Thursday. In the meantime, all customers can now track prices and receive alerts for 16 cryptocurrencies on the app.

The prices of bitcoin and other digital currencies surged last year as investors, including some on Wall Street, bet cryptocurrencies and the blockchain technology behind them could one day transform the world as much as the internet has. Bitcoin was developed as a way to send money around the world quickly and cheaply. However, transaction fees have soared above $20 and other digital currencies have yet to prove their actual use.

That hasn't stopped investors from pouring in.

"Cryptocurrencies have become the first foray into investing and financial services to a large number of people," Robinhood co-founder Vlad Tenev told CNBC in a phone interview. "Now it's become more and more clear [that bitcoin is an] investing asset."

That's a change from October, when Tenev told CNBC that Robinhood was focused on expanding its stock trading business.

Still, "I wouldn't call it a shift," Tenev said. "We're not seeing significant impact from cryptos on the size of the equity market. Marketwide, trading is increasing, investing is increasing."