Richard Branson, the entrepreneur and adventurer who founded The Virgin Group, has become the latest investor in mobile payment firm Square Inc. He made the investment as a private individual, and the amount has not been disclosed. The company recently closed a $100 million financing round led by Kleiner Perkins Caufield and Byers.

Square distributes a card reader and mobile application that enable anyone to accept credit card payments for goods and services. It reports having shipped more than 800,000 card readers to merchants since it went live in 2010 and said it currently is processing over $2 billion in payments annually.

Branson said it was the egalitarian nature of the product that appealed to him. “I’m very passionate about helping people start and grow successful businesses, and Square is an incredible technology that inspires and empowers everyone to be an entrepreneur,” he said.

The news broke when Square co-founder and chief executive officer Jack Dorsey tweeted it, according to the Wall Street Journal. Dorsey also co-founded Twitter. Branson confirmed his investment shortly later on his own Twitter feed.

Square plans to expand beyond its United States roots during 2012. Square for iPad serves as a full point of sale system for businesses to accept payments, track inventory, and share menu and location information, while its free credit card reader works with iPhone, iPad, and Android devices. Earlier this month the company updated its Square Card Case, an app that recognizes when a user is within 100 meters of a business that accepts it and automatically opens up a digital tab that the customer can pay just by giving their name in lieu of cash or credit card.

Related links:

Press release [PDF] –

Virgin’s statement –

Reuters –

Wall Street Journal –

Photo of Richard Branson by Flickr user NRKbeta, used under Creative Commons license