The 7-storey, 65,000 sq ft Endeavour building will become the South Yorkshire base for up to 1,000 staff across the company’s functions, including its business and consumer divisions.
Staff from across the company have begun moving into the building, based at Sheffield's Digital Campus.
The building will also become the primary base for employees from Sheffield-founded company Plusnet, which has been part of BT Group since 2006.
BT unveiled its plans to take up residence at Endeavour in September 2022, with developers formally handing over the building to the company in August last year.
The building, which has been fitted out with the latest equipment, features collaborative workspaces and a rooftop terrace. Endeavour also forms part of BT Group's Better Workplace Programme.
Brent Mathews, property director at BT Group, said, "Sheffield is an important strategic location for BT Group, and opening this office in the heart of the city allows us to provide a state-of-the-art building for our colleagues based here.
"Our Better Workplace Programme was designed to ensure our colleagues work in future-fit buildings they can be proud of and Endeavour embodies that perfectly.
"While the past few years have seen changes to work patterns and increased flexibility, offices remain an integral part of our way of working, allowing us to bring colleagues together to collaborate, and to support economic growth in our key locations."
The opening of the new Sheffield site follows the multi-million pound refurbishment of BT Group's Doncaster contact centre, which was completed in 2021.
BT Group employs more than 6,700 people across the region, including around 1,900 in Sheffield.