“But if a big merchant jumping into some mobile payment solution signals to other merchants that there is an opportunity here,” he added, “that might change the psychology for other merchants.”

Starbucks is also investing $25 million in Square as part of its latest round of financing, which values the company at $3.25 billion, and Howard D. Schultz, Starbucks’s chief executive, will join Square’s board.

Starbucks has offered its own mobile payment app since last year and processes more than a million mobile payments a week. Customers will continue to be able to use it, but they will also be able to use Pay With Square, Square’s cellphone app, which eliminates even having to take the phone out of your pocket or sign a receipt.

At first, Starbucks customers will need to show the merchant a bar code on their phones. But when Starbucks uses Square’s full GPS technology, the customer’s phone will automatically notify the store that the customer has entered, and the customer’s name and photo will pop up on the cashier’s screen. The customer will give the merchant his or her name, Starbucks will match the photo and the payment will be complete.