The total area of the city is 793.29 square kilometres (306.29 sq mi) Niimi is located on a karst plateau in the Chugoku Mountains in northeast Okayama Prefecture.

Due to the rugged terrain, there are many waterfalls, dams, hot springs and multiple limestone caves in the city.

The main urban area is located on the upper reaches of the Takahashi River (110.7 kilometres (68.8 mi).

It appears in documentary records as early as the Heian period (794–1185) when a shōen manorial estate called Niimi-shō' occupied much of the area of present-day Niimi.

During the early Edo Period, it was part of the holdings of Bitchū-Matsuyama Domain and developed as an important center of inland trade, due to its connection by the Takahashi River with the jōkamachi of Matsuyama.

[4] In 1697, the Tokugawa Shogunate established Niimi Domain, which was ruled by the Seki clan to the Meiji restoration.

[3] Niimi has a mayor-council form of government with a directly elected mayor and a unicameral city legislature of 16 members.

In terms of national politics, the city is part of Okayama 3rd district of the lower house of the Diet of Japan.

[4] The principle industries in modern Niimi are limestone quarrying, cement production,[10] forestry, and tourism.