Shimōsa Province (下総国, Shimōsa no Kuni; Japanese pronunciation: [ɕi.moꜜː.sa (no kɯ.ɲi), ɕi.moː.saꜜ-]) 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 (the parts that used to be located east of the lower reaches of the old Tone River prior to the river's eastward diversion, i.e. the parts of the former Katsushika District of Shimōsa that have been transferred to North Katsushika District of Saitama Prefecture and Sumida, Kōtō, Edogawa, and Katsushika wards of 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).
Shimōsa is classified as one of the provinces of the Tōkaidō. It was bordered by Kazusa Province to the south, Musashi and Kōzuke Provinces to the west, and Hitachi and Shimotsuke Provinces to the north. Under the Engishiki classification system, Shimōsa was ranked as a "great country" (大国) and a far country (遠国).
