Company officials have also hinted at interest in acquisitions in Europe.
"This deal will let us monetize our towers while giving us the ability to add capacity as we need it," AT&T Senior Vice President Bill Hogg said in a statement. "We'll get additional financial flexibility to continue to invest in our business and maintain a strong balance sheet and return value to our shareholders."
AT&T said it doesn't expect the deal to have any significant impact on its financial results. It said last month it was close to making a deal for its towers.
The company will sublease capacity on the towers for $1,900 per month per site, with annual rent increases of 2 percent per year, the statement said. The company will have access to reserve capacity on the towers for future use.
After the leases expire, Crown Castle can buy the towers for about $4.2 billion. The average lease runs for about 28 years.
It's not the first big tower purchase Crown Castle has made from wireless operators. In October 2012, the company sold $1.65 billion in debt to help fund a $2.4 billion purchase about 7,200 towers from T-Mobile USA Inc.
Crown Castle said the towers involved in the AT&T deal are located in top markets across the U.S. It says nearly half the AT&T towers are in the nation's top 50 markets. The company has been focusing on the nation's top 100 markets, where it expects most wireless network activity and upgrades to occur.
When the deal closes, Crown Castle will have about 40,000 towers through the U.S., keeping it the largest wireless infrastructure operator in the country, it said in a statement.
The company has scheduled a conference call for 7:30 a.m. EDT Monday to discuss the transaction and its third-quarter results.