ADTRAN, Inc. is a provider of telecommunications networking equipment and internetworking products. Its headquarters are in Huntsville, Alabama.

The company is ISO 9001 and TL9000 certified. ADTRAN was a NASDAQ-100 Index stock from 1996 to 1998. It was #85 on BusinessWeek's 100 "Hot Growth Companies" list in 2006.

ADTRAN was founded by Mark C. Smith and Lonnie McMillian, and began operations in 1986. The AT&T divestiture of the Regional Bell Operating Companies (RBOCs), with restrictions that effectively barred their manufacturing of equipment, created an opportunity for companies such as ADTRAN to supply network equipment, both to the seven RBOCs and the new and independent telephone companies in the United States.

ADTRAN adapted telephony technology for use in enterprise-wide area networks. ADTRAN has become a supplier of local loop access and deployment products for fiber, DS3, T1/E1, wireless T1/E1, Digital Subscriber Line (HDSL, HDSL2, HDSL4, SDSL, SHDSL), Frame Relay, Asynchronous Transfer Mode (ATM), Integrated Services Digital Network (ISDN), and Digital Data Service (DDS) digital services. ADTRAN supplies multi-service access platforms and Integrated Access Devices (IADs) for converged voice and data networks.

According to Computer Business Review, ADTRAN enhanced its portfolio of carrier ethernet products with the addition of three new multi-service modular routers supporting a services-enabled sustained throughput of up to 250Mbit/s.

In December 2011 ADTRAN announced a plan to buy the Broadband part from Nokia Siemens Networks. In 2012 ADTRAN’s closing of acquisition of the Nokia Siemens Networks Broadband division was finished.

ADTRAN's U.S. corporate headquarters is located in Huntsville, Alabama, in the second largest research park in the United States. ADTRAN's European Regional Headquarters is in Munich, Germany. ADTRAN has engineering research and development centers in:

Sales and support offices are located throughout the U.S. and around the world.

Competitors of ADTRAN's Carrier Networks Division include Alcatel Lucent (presently Nokia), Cisco Systems, Fujitsu, Huawei, and Tellabs. There are also a number of smaller, specialized firms with which compete with ADTRAN, such as Actelis, Calix, Zhone, and other privately held firms.

Competitors of ADTRAN's Enterprise Networks Division include Cisco Systems, Avaya, Nokia, Hewlett Packard, Enterasys Networks, Extreme Networks, Aruba, Aerohive, Meraki, and other smaller companies. Some of these companies compete in a single product segment while others compete across multiple product lines.