1seg (ワンセグ, wansegu^?) is a mobile terrestrial digital audio/video and data broadcasting service in Japan, Argentina, Brazil, Chile and Peru. Service began experimentally during 2005 and commercially on April 1, 2006. In Brazil, the broadcast started in late 2007 in just a few cities, with a slight difference from Japanese 1seg: a 30 frame/s transmission, while in Japan they use a 15 frame/s transmission. The first mobile phone handsets for 1seg were sold by KDDI to consumers in autumn 2005.

ISDB-T, the terrestrial digital broadcast system used in Japan, Argentina, Brazil, Chile and Peru, is designed so that each channel is divided into 13 segments, with a further segment separating it from the next channel. An HDTV broadcast signal occupies 12 segments, leaving the remaining (13th) segment for mobile receivers. Thus the name, "1seg" or "One Seg".

The ISDB-T system uses the UHF band at frequencies between 470 and 770 MHz (806 MHz in Brazil), giving a total bandwidth 300 MHz. The bandwidth is divided into fifty channels name channels 13 through 62. Each channel is 6 MHz wide consisting of a 5.57 MHz wide signalling band and a 430 kHz guard band to limit cross channel interference. Each of these channels is further divided into 13 segments, with each with 428 kHz of bandwidth. 1 seg uses a single of these segments to carry the 1seg transport stream.