AT&T acquired BellSouth Corp. late last year, and it was part of their merger plan to sell all 2.5 GHz spectrum licensed to or leased by BellSouth.
The sale to Clearwire includes Broadband Radio Service spectrum, which is used for commercial delivery of high-speed wireless broadband services.
It also includes Education Broadband Service (EBS) spectrum. Groups holding EBS licenses may lease portions of their spectrum to commercial operators.
The spectrum sale is expected to close promptly, following government approvals, AT&T said.
The move comes a week after Clearwire, which builds and operates wireless broadband networks, set its planned initial public offering at 20 million shares for $23 to $25 per share.
The IPO is expected to bring the company between $460 million and $500 million in proceeds and help it boost its range of mobile communications services.