The companies said they plan to offer consumers with wireless Internet access at speeds comparable to wireline broadband services. The two had previously planned to build their networks separately.
Kirkland, Wash-based Clearwire, which saw its stock price jump 30% in Thursday's late trading, was founded by wireless pioneer Craig McCaw.
Last year, Nextel said that it planned to build its own nationwide broadband wireless network, but said on Thursday that it will now partner with Clearwire "to increase capital efficiency and reduce overall network development and operating costs."
The companies said the agreement is subject to review by the Department of Justice, and approval of spectrum license assignments and transfers by the Federal Communications Commission.