Shimōsa Province (下総国, Shimōsa no Kuni) 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).

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).

Following the installation of Tokugawa Ieyasu in Edo, after the Battle of Odawara, he created eleven han within the borders of Shimōsa to reward his followers, with the remaining area retained as tenryō territory owned directly by the shōgun and administered by various hatamoto.

Following the Meiji Restoration, these various domains and tenryō territories were transformed into short-lived prefectures in July 1871 by the abolition of the han system.

The area of former Shimōsa Province was organized into twelve districts by the Meiji cadastral reforms: Chiba, Inba, Katori, Kaijō, Shimohabu.