Shimōsa Province was a province of Japan in the area of modern Chiba Prefecture and Ibaraki Prefecture as well as the bordering parts of Saitama Prefecture and Tokyo. It lies to the north of the Bōsō Peninsula (房総半島), whose name takes its first kanji from the name of Awa Province and its second from Kazusa and Shimōsa Provinces. Its abbreviated form name was Sōshū (総州) or Hokusō (北総).

The Bōsō Peninsula is a peninsula that encompasses the entirety of Chiba Prefecture on Honshu, the largest island of Japan. It is part of the Greater Tokyo Area. It forms the eastern edge of Tokyo Bay, separating it from the Pacific Ocean. The peninsula covers approximately 5,034 square kilometres (1,944 sq mi).