Starbucks, the largest coffee-shop chain, will use Square Inc.’s payment system in 7,000 stores and invest $25 million in the startup, making it easier for customers to make purchases with mobile phones.

Starbucks CEO Howard Schultz will join the board of Square, which was started in 2009 by Twitter co-founder Jack Dorsey. Square’s software lets merchants accept electronic payments on their own hardware or through a device that plugs into a phone or tablet.

Starbucks is marrying its own mobile-phone payment program with Square’s transaction-processing tools, angling to make it as easy as possible to buy a latte at its U.S. stores. The investment is part of $200 million in funding that gives San Francisco-based Square a valuation of $3.25 billion, a person with knowledge of the matter said.

“Every tech company is trying to crack the code and chase down how to monetize their business in regard to mobile and mobile payments,” Schultz said in an interview. “It’s the fastest-growing opportunity we’ve ever seen in terms of customer acceptance.”

Starbucks already processes more than 1 million mobile purchases a week in the U.S. through its smartphone app and has logged more than 60 million since its introduction, Schultz said.

The app, available for Apple’s iPhone and Google’s Android software, lets users buy coffee and other items with a Starbucks card by scanning a barcode on the screen of the phone.

Square’s software will similarly let Starbucks customers pay by scanning an enabled phone, only now Square will process the payments, Dorsey said.

“We’ve always built the company and the technology around the idea of it being used by an individual all the way up to the largest organization in the world,” Dorsey said in an interview. “It should scale effortlessly between that range.”