Verizon will be anchor tenant of the The Hub On Causeway, leasing nearly 439,000 square feet on the 15th through 31st floors of the office tower rising next to TD Garden.

A Verizon spokesman declined to give specifics this week about how many employees and which divisions will be based at the new offices. But commercial real estate sources have said the expansion is by Verizon’s year-old digital subsidiary Oath Inc., which is the parent of the Yahoo, TechCrunch and HuffPost sites.

The transaction is the largest office lease in Boston this year. The space could house approximately 2,500 employees, based upon an allocation of 175 square feet per employee. Verizon is expected to occupy the tower beginning in June 2021, according to a notice of lease filed Monday with the Suffolk County Registry of Deeds.

Developer Boston Properties redesigned the office tower this spring, citing changes in tenants’ preferences since the original approvals in 2013 for the 1.9-million-square-foot mixed-use development.

The Gensler-designed tower will include two terraces and a covered roof deck, and seven two-story “porches” with operable windows. The porches will provide “innovative and flexible work space” or amenity areas, Boston Properties said in a submission to the Boston Civic Design Commission this spring. The commission approved the redesign May 1.

The remaining office space in the tower is reportedly spoken for, with marketing agency Epsilon mentioned as a potential tenant.

Online security company Rapid7 will occupy 157,061 square feet in the office podium building scheduled for completion this winter at The Hub on Causeway. The first phase of the project also includes a Star Market, Live Nation concert venue, 15-screen ArcLight Boston Cinema, restaurants and a food hall.

Verizon acquired Yahoo! Inc. for $4.5 billion in June 2017 and spun out Oath as a new subsidiary for its more than 50 media and technology brands. The company is headed by former AOL CEO Tim Armstrong, who founded a financial publication in Cambridge during the 1990s and is a former owner of the Boston Blazers indoor lacrosse team.