AT&T Inc. is an American multinational conglomerate holding company headquartered at Whitacre Tower in Downtown Dallas, Texas, and incorporated in Delaware. It is the world's largest telecommunications company, the largest provider of mobile telephone services, and the largest provider of fixed telephone services in the United States through AT&T Communications. Since June 14, 2018, it is also the parent company of mass media conglomerate WarnerMedia, making it the world's largest media and entertainment company in terms of revenue. As of 2018, AT&T was ranked #9 on the Fortune 500 rankings of the largest United States corporations by total revenue.

AT&T began its history as Southwestern Bell Telephone Company, a subsidiary of the Bell Telephone Company, founded by Alexander Graham Bell in 1877. The Bell Telephone Company became the American Telephone and Telegraph Company in 1885 and was later rebranded as AT&T Corporation. The 1982 United States v. AT&T antitrust lawsuit resulted in the divestiture of AT&T Corporation's ("Ma Bell") subsidiaries or Regional Bell Operating Companies (RBOCs), commonly referred to as "Baby Bells", resulting in several independent companies, including Southwestern Bell Corporation; the latter changed its name to SBC Communications Inc. in 1995. In 2005, SBC purchased its former parent AT&T Corporation and took on its branding, with the merged entity naming itself AT&T Inc. and using its history, iconic logo and stock-trading symbol. AT&T Inc. acquired BellSouth in 2006, the last independent Baby Bell company, making its formerly joint venture Cingular Wireless (which had acquired AT&T Wireless in 2004) wholly owned and rebranding it as AT&T Mobility.

The current AT&T reconstitutes much of the former Bell System, and includes ten of the original 22 Bell Operating Companies along with the original long-distance division.