Visa in a regulatory filing revealed that it owns about 4.19 million of Square’s Class B shares, which don’t trade publicly.
But the credit card company has the option to convert those shares to about 3.52 million Class A shares, which would give it a 9.99% stake in the publicly-traded share class.
After mutual fund giant Capital Research & Management, Visa is now the second-largest owner of Square’s A shares.
Visa first bought a stake in the mobile-payments startup Square back in April 2011, but terms of that investment were never revealed.
Square, which is co-founded by Twitter CEO Jack Dorsey, made market debut in November 2015. It makes a plastic device that allows mobile phones to accept credit card payments.