The deal includes all NII's mobile assets in Mexico, including spectrum licenses, network facilities, retail stores and 3 million customers, AT&T said in a press release.
NII, based in Reston, Virginia, filed for bankruptcy in U.S. court in September. The company also operates mobile networks in Argentina and Brazil.
The deal with AT&T, subject to approval by the U.S. Bankruptcy Court for the Southern District of New York and regulatory approvals in Mexico, is expected to close by mid-2015, NII said in a press release.
The deal will allow AT&T to combine Nextel Mexico with Iusacell, another Mexican mobile provider that AT&T acquired in November for $2.5 billion. Iusacell, with about 9 million customers, was the third largest mobile carrier in Mexico at the time.
Mexico City-based América Móvil's Telcel is the dominant mobile carrier in Mexico, with a market share of about 70 percent.
Nextel Mexico's network covers about 76 million people. The acquisition of Nextel Mexico supports AT&T's plans to offer more competitive services and faster mobile broadband speeds in Mexico, AT&T said.