Shimōsa Province (下総の国 Shimōsa no Kuni?) was a province of Japan in the area modern Chiba Prefecture, and Ibaraki Prefecture. 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).
Shimōsa was originally part of a larger territory known as Fusa Province (総国, occasionally 捄国 Fusa-no-kuni?), which was divided into "upper" and "lower" portions (i.e. Kazusa and Shimōsa) during the reign of Emperor Kōtoku (645–654). It was well-known to the Imperial Court in Nara period Japan for its fertile lands, and is mentioned in Nara period records as having supplied hemp to the Court. Shimōsa was divided into 11 (later 12) counties. The exact location of the capital of Shimōsa is not precisely known, but is believed to have been somewhere within the borders of the modern city of Ichikawa, Chiba, near Kōnodai Station where the ruins of the Kokubun-ji have been located. However, the Ichinomiya of Shimōsa Province is the Katori Jingū in what is now the city of Katori, Chiba, on the opposite coast of the province.