SunTrust Robinson Humphrey, an investment banking and financial services firm, is doubling its space in Buckhead’s financial district.

The firm has decided to stay in the Atlanta Financial Center, where it has been since partners in its predecessor company helped develop the complex in the 1980s.

The firm is adding nearly 140,000 square feet to its already sizable lease, making it one of the largest tenants in Buckhead’s financial district. Its new lease will total nearly 230,000 square feet and won’t expire for another 11 years.

The firm is growing, said Hugh S. “Beau” Cummins, SunTrust Robinson Humphrey’s president and CEO.

“As the largest tenant in the building,” he said, “we look forward to remaining a key part of Atlanta’s vibrant financial district.”

Owner Hines, a real estate firm based in Houston, acquired the complex in 2001 in partnership with a pension fund. The owners had another recent win when the law firm Morris Manning & Martin decided not to move to the Campanile Building in Midtown, as it had announced. The firm opted to stay in about 120,000 square feet at the Atlanta Financial Center. Morris Manning & Martin, with about 300 employees, has been in the building since 1987.

At more than 900,000 square feet, the Atlanta Financial Center is now 84 percent leased, Hines said. And despite newer office space in the Buckhead market, the complex has held its own. Other leases it added or renewed include the DeMoss Group, Heineken, Massachusetts Mutual Life Insurance, the North American Electric Reliability Corp. and Prudential Insurance.