Chiba Prefecture Sanmu has a humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light to no snowfall.

Sanmu has a mayor-council form of government with a directly elected mayor and a unicameral city council of 20 members.

In terms of national politics, the city is part of Chiba 11th district of the lower house of the Diet of Japan.

Small-scale agriculture in Sanmu produces a variety of fruits and vegetables, but the town is noted for carrots, watermelon, peanuts.

[5] Sanmu serves as a bedroom community for nearby Chiba and Tokyo, and housing development continues in the city.