KDDI Corporation (KDDI株式会社, KDDI Kabushiki Gaisha^?) (TYO: 9433) is a Japanese telecommunications operator formed in October 2000 through the merger of DDI Corp., KDD Corp., and IDO Corp. It has its headquarters in the Garden Air Tower in Iidabashi, Chiyoda, Tokyo.

KDDI provides mobile cellular services using the "au by KDDI" brand. ISP network and solution services are provided under the au one net brand, while "au Hikari" is the name under which long-distance and international voice and data communications services and Fiber to the Home (FTTH) services are marketed. ADSL broadband services carry the brand name "ADSL One", and IP telephony over copper is branded as "Metal Plus".

On April 1, 2002, au by KDDI launched 3G networks using CDMA2000 1x technology.

On November 28, 2003, au by KDDI launched EV-DO Rev 0 service in the "CDMA 1X WIN" brand, and revolutionized Japan's mobile telecommunication industry by introducing fixed rate data subscription plans at a data rate of 2.4 Mbit/s.

On December, 2006, au by KDDI became the first carrier to provide an EV-DO Rev A service at a data rate of 3.1Mbit/s (downlink), 1.8Mbit/s (uplink).