Kasai is located almost in the center of the Harima Plain, with forests in the north and low mountains in the south.

Hyōgo Prefecture Kasai has a Humid subtropical climate (Köppen Cfa) characterized by warm summers and cool winters with light to no snowfall.

In the Edo Period, it was mostly tenryō territory administered directly by the Tokugawa shogunate with a small portion of the area under the control of Himeji Domain.

Kasai has a mayor-council form of government with a directly elected mayor and a unicameral city council of 15 members.

In terms of national politics, the city is part of Hyōgo 4th district of the lower house of the Diet of Japan.