Room 641A is a telecommunication interception facility operated by AT&T for the U.S. National Security Agency, part of a mass surveillance program. It began operations in 2003 and was revealed by AT&T technician Mark Klein in 2006.

Located in the SBC Communications building at 611 Folsom Street, San Francisco, the room measures about 24 by 48 feet and contains equipment including a Narus STA 6400 to intercept Internet communications. According to J. Scott Marcus, it has access to all Internet traffic passing through the building.

The Electronic Frontier Foundation filed a class-action lawsuit, Hepting v. AT&T, on January 31, 2006. The case was dismissed in 2011 after Congress granted retroactive immunity. A separate case, Jewel v. NSA, was filed in 2008; in 2019, the court found Klein could not establish the room's function based on his own knowledge. Over 50 other lawsuits were filed in 2006.