AT&T said it will use the additional spectrum for "furthering its 4G LTE deployment and providing additional capacity and enhanced network performance for customers’ growing mobile Internet usage."
AT&T will also be responsible for Leap's $2.8 billion in debt.
The deal is subject to approval by the Federal Communications Commission and the Justice Department. AT&T said it expects to close the transaction in six to nine months.
The announcement comes two years after AT&T's unsuccessful attempt to buy T-Mobile for $39 billion. The FCC and Justice Department blocked that merger after concluding it would have stifled competition in the wireless industry and led to higher prices for consumers.