WeWork is moving into the Back Bay.

The fast-growing co-working space operator has signed a lease on a new location and plans to be up and running by the end of the year in two floors of the Park Square Building at 31 St. James Ave.

It will be the third Boston location for the company, which leases flexible office space to freelancers, startups, and — increasingly — small outposts of larger companies. It opened offices in Fort Point Channel and the Leather District near South Station in 2014.

The Back Bay, with its good transit access and dense collection of businesses, is a logical next step, said Dave McLaughlin, who runs WeWork’s operations in Boston and other East Coast cities.


“Our model runs off of density and innovation,” he said. “That neighborhood makes sense.”

WeWork currently has about 1,900 members, in 450 companies, working at its two other Boston sites. Some of those people may shift to the Back Bay if they find it more convenient, McLaughlin said.

There’s room to grow, he said, adding small companies and freelancers from the neighborhood as well as suburban companies that want a foothold in central Boston to better connect with potential employees, partners, and clients.

“This Back Bay building offers an opportunity for divisions of larger companies, too,” McLaughlin said. “It’s that mix of company sizes that makes this work.”

The opening date will depend in part on how long it takes to build out the space, which covers two floors of a block-long building between Arlington and Berkeley streets.

And the company — which has been on an expansion tear of late in big cities across North America — says it probably isn’t done growing in Boston. They’re scouting more locations, McLaughlin said.

“We have a bullish outlook on Boston,” he said. “We absolutely plan to do more. We just don’t have anything locked down just yet.”


Tim Logan can be eeached at tim.logan@globe.com. Follow him on Twitter @bytimlogan.