Tosa (土佐国; -no kuni) was an old province of Japan in the area that is today Kochi prefecture on Shikoku. Tosa bordered on Iyo and Awa provinces.

The ancient capital was near modern Nankoku. Tosa was ruled by the Chosokabe clan during the Sengoku period, and Chosokabe Motochika briefly unified Shikoku under his rule, although he was reduced to Tosa again by Toyotomi Hideyoshi and dispossessed entirely after Sekigahara. Tosa was a relatively poor province, and lacked a strong castle town even under the Chosokabe. After Sekigahara, the castle town of Kochi was established and remains the main city to this day.