In a deal marking the largest lease transaction for downtown Boston in four years, Bingham McCutchen has signed an agreement for a new headquarters near its current location.

The Boston-based law firm signed a 15-year lease for about 320,000 square feet of space at One Federal now occupied by Fidelity Investments. Bingham’s current Boston office is at 150 Federal, where it has been for nearly 20 years. The new office, which spans 13 floors, will be roughly 80,000 square feet larger and constitutes the largest such deal for the downtown area since 2001, according to the firm.