A number of Cash users noticed the new feature, and took to Twitter to voice their surprise.
Users were able to swipe right from their Cash Card page to buy bitcoin.
The feature also shows the current price of bitcoin against the U.S. dollar.
A Square spokesperson confirmed that the firm was piloting the feature with a small number of Cash users.
The spokesperson said: "We've found that (our customers) are interested in using the Cash app to buy bitcoin. We're exploring how Square can make this experience faster and easier, and have rolled out this feature to a small number of Cash app customers.
"We believe cryptocurrency can greatly impact the ability of individuals to participate in the global financial system and we're excited to learn more here."