Shimo-Usa province (下総国 -no kuni) was an old province of Japan located in the northern part of modern Chiba prefecture on the island of Honshu.

Shimosa bordered on Hitachi, Kazusa, Kozuke, Musashi and Shimotsuke Provinces. Its capital was Sakura.

The ancient provincial capital was near modern Ichikawa, although in feudal times Sakura was a more important center. Shimosa passed through various hands, and at times all or portions of it were ruled by stronger daimyo from Musashi or Kazusa.