It's a generous 88 percent premium to Leap's closing price, but it may not be enough to close the deal.
Leap shares more than doubled in after-hours trading, hitting $16.80 per share almost an hour after the announcement.
That's about 11 percent above AT&T's purchase price, indicating investors believe another bidder may emerge with a higher offer.
At the close, Leap's stock had already surged 37 percent from its July 24 price of $5.83.
The company has been the subject of takeover speculation for several years.
John Paulson's investment firm holds a large stake: 7.8 million shares, about 10 percent of the shares outstanding.
Headquartered in San Diego, Leap has approximately 5 million subscribers in 35 U.S. states. It also had $2.8 billion of net debt as of April 15, 2013.AT&T plans to retain Leap's "Cricket" brand name and expand its presence in the U.S. Leap operates a 3G CDMA network and a 4G LTE network covering 21 million people. After the deal closes, AT&T said it plans to use Leap's spectrum to help develop its 4G LTE network.
In its news release, AT&T said, the resulting company will provide "increased competition, better device choices, improved customer care and a significantly enhanced mobile Internet experience for consumers seeking low-cost prepaid wireless plans."
The deal is the latest in a series of acquisitions in the wireless industry and follows Softbank's acquisition of Sprint.
As the No. 2 wireless provider in the United States, AT&T has been looking to grow in a mature market. It had held talks to buy Leap last year, Reuters reported.
The acquisition is subject to review by the FCC and Justice Department. AT&T expects to close the transaction in six to nine months.
Reuters contributed to this story