The Appalachian Mountain Club has acquired 10 City Square in Charlestown and will relocate from its longtime headquarters on Beacon Hill next fall.

The outdoors group sold its 3-5 Joy St. property in September for $15 million to a development group. It reinvested the proceeds in the $13.1 million acquisition of the Charlestown property, which contains 38,442 square feet of office space. The seller was Carey Realty LLC and City Square Holdings LLC.

AMC, which has 70 full-time employees based in Boston and 92,000 members, will occupy half of the building with the remainder leased to existing tenants including Legal Oysteria, a seafood restaurant. Building and technology upgrades are planned to enable the group to expand its programs for city residents, including its Youth Opportunities Program which provides outdoor leadership training to social workers and teachers.

Appalachian Mountain Club CEO John Judge said the group looked at more than 40 properties in Boston near transit hubs after outgrowing its 22,000-square-foot home of nearly a century. The Charlestown property is a half-mile from North Station and the Community College stop on the MBTA’s Orange Line.

AMC originally considered leasing office space but decided to buy a property because of high rent costs, Judge said.

“Looking at sites, it became apparent to us the lease was going to be outrageous and the economics of buying for a group this large made sense,” Judge said. “It also puts us in the driver’s seat to expand in the decades ahead.”

Boston-based McCall & Almy represented the club in its search and will contribute a portion of related fees back to AMC. Goodwin Procter’s Michael Litchman provided pro bono legal counsel.