Map of Japanese provinces (1868) with Shimōsa Province highlighted
Shimōsa Province (下総国 Shimōsa no Kuni) was an old province of Japan in the area of Chiba Prefecture and Ibaraki Prefecture on the island of Honshū. It was also known as Sōshū (総州) or Hokusō (北総).
The province had borders with Kazusa Province to the south, Musashi and Kōzuke Provinces to the west, and Hitachi and Shimotsuke Provinces to the north.
View of Shimōsa Province, woodblock print by Hiroshige, 1856-1858
Shimōsa was part of a larger territory known as Fusa Province (総国 or 捄国 Fusa-no-kuni). Fusa which was divided into Kazusa and Shimōsa during the reign of Emperor Kōtoku (645-654).
In the Meiji period, the provinces of Japan were converted into prefectures. The maps of Japan and Shimōsa Province were reformed in the 1870s.
Katori jinja was the chief Shinto shrine (ichinomiya) of Shimōsa. 
Media related to Shimosa Province at Wikimedia Commons