(CNN)-- Square. Everywhere you want to be?
Visa, the credit card processing giant famous for that marketing slogan, disclosed in a SEC filing late Thursday that it now owns a 9.99% stake in the class A shares of Square, the mobile payments startup co-founded by Twitter CEO Jack Dorsey.
Shares of Square surged nearly 10% on the news at one point Friday before pulling back.
Visa already had an investment in Square. It bought a stake in the company back in April 2011. But terms of that investment were never disclosed.
Visa said in its filing it has 4.2 million class B shares but that only 3.5 million of those shares can convert into the A shares that actively trade on the market.
As a result, Visa is now the second-largest owner of Square's A shares -- trailing only mutual fund giant Capital Research & Management.
Square also has many more Class B shares though that are controlled by Dorsey and early backers like venture capital firms Khosla Venture and Sequoia Capital as well as co-founder James McKelvey. So Square's overall stake in the company is closer to 1.1%.
Still, the investment could be viewed as major validation for Square, whose stock has slumped since its market debut last November.
And that's despite the fact that Square priced its offering at a discount to what it was valued at as a private company in an attempt to lure investors.
There have been concerns that Square is in too crowded a field.
In addition to competition from the likes of Visa and its top rival MasterCard, Square also has to go up more established payment processing firms like First Data as well as PayPal.
Square may also face more competition from tech giants Alphabet, Facebook, Amazon and Apple -- which would all love to get a bigger piece of the growing mobile payments market.
Apple is even rumored to be considering using its Apple Pay to launch a peer-to-peer money transfer service that could rival Square's Cash and PayPal's Venmo.
Some investors have also expressed worries about Dorsey's ability to run both Square and Twitter -- which requires a lot of his time given its troubles. Shares of Twitter hit an all-time low Thursday after it reported a surprise drop in monthly active users.
Visa pointed out Friday that it had not bought more Square since the IPO. It was merely disclosing its existing stake. Square had no comment.
But it is clear that Visa also wants to be a bigger player in mobile -- and a deeper partnership with Square could help give Visa a leg up on MasterCard.
Visa announced Friday that is planning to show off new payment technologies at the influential Mobile World Congress trade show in Barcelona later this month.
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