The retail giant has offered free Wi-Fi access in its stores since 2003, but it was previously provided by T-Mobile.
However, BT confirmed that it has taken over the deal. It said it will begin installing its service in stores this week, and plans to roll out the service to all 650 UK stores by the end of the summer.
The Starbucks Wi-Fi hotspots will fall under BT's OpenZone brand, allowing them to be used by Apple iPhone owners and BT OpenZone customers.
Chris Bruce, CEO, BT Openzone said: "Starbucks is the perfect place to work or catch up over email. Use of our Wi-Fi doubled last year and coffee shops are increasingly popular as a third place [after home and the office] to work or log on to the internet. We're constantly looking to boost the choice of places to connect - where people want it. The location of Starbucks stores in the high street and travel hubs is helping to extend the reach of Wi-Fi to customers."