The Sōbu Main Line (総武本線, Sōbu-honsen^?) is a Japanese railway line operated by the East Japan Railway Company (JR East) in Japan. It connects Tokyo with the east coast of Chiba Prefecture. It runs through the cities of Funabashi, Chiba, and Chōshi. Its name derives from the old provinces of the area which it serves: Musashi (武蔵国), Shimōsa (下総国) and Kazusa (上総国). Its official line color is yellow.

The character of the line changes at Chiba. The western, urbanized section is informally, but commonly, called the Sōbu Line (総武線, Sōbu sen^?) without using "Main". The "Main Line", in popular usage, refers to the trains operated east of Chiba on the line.

Signs at stations and in trains, and vocal announcements in trains, both maintain this distinction: with Main for the eastern rural section; without Main for the western frequent travel zone.

Local trains run between Ochanomizu and Chiba Station and are called the Chūō-Sōbu Line (中央・総武線^?) or Sōbu-Local Line (総武緩行線, Sōbu-Kankō^?). Rapid trains are also operated from Tokyo Station to Chiba Station and use different tracks from the local service trains. The rapid trains are called Sōbu Kaisoku Line (総武快速線, Sōbu Kaisoku sen (Sōbu Rapid Line)^?). For the most part, these trains use the same track, except when they reach Tokyo: Sōbu-Kankō trains continue onto the Chūō Line, while Sōbu-Kaisoku trains continue onto the Yokosuka Line. The following sections describe trains east of Chiba. For details west of Chiba, see the relevant articles.

Mainline, Main line, or Main Line may refer to: