The "Square of Europe" just picked up some big U.S. backers.

Stockholm-based mobile payment company iZettle announced this week that it has closed a 25 million euro ($31.4 million) series B funding round.

The investment was led by Silicon Valley venture capital firm Greylock Partners and European tech investment firm Northzone, but MasterCard was also involved in the funding round.

To date, iZettle has simply helped create a new revenue stream for the credit card company: Using a credit card reader that plugs into iPads and iPhones, the mobile payment solution has allowed Northern European small business owners that previously accepted cash only to begin taking credit card payments.

But with this investment and other projects, MasterCard says that it is hoping to take a more active role in the growing mobile payment industry.

"Mobile phones are consumer-based devices. They were not built for payments specifically, and there is guidance needed in that area," says John Verdeschi, MasterCard's senior business leader of mobile product development. "There are various players in the space, and we are committed to partnering with all of them"

One reason MasterCard might be interested in the mobile payment device: iZettle charges 2.75% for each transaction processed. Verdeschi would not comment on how much of each transaction is given to the credit card companies.

In the funding announcement, Laurel Bowden, a Greylock partner who will join iZettle's board of directors, says iZettle is the only smartphone-based mobile commerce offering that satisfies all international security requirements.