Obi Castle, Japanese castle in Nichinan, Japan

Obi Castle is a hirayama-style castle on the Shirasu plateau on the north bank of the Sakatani River in Nichinan, Miyazaki Prefecture. Its enclosures are cut into the plateau, whose vertically cut walls serve as fortifications, with moats and stone walls added. The present gate, restored in 1978, has a white plastered upper wall, a dark tiled roof and two large wooden doors, and stands between rough stone walls.

Obi Castle is said to have been built by the Tsuchiya clan in the Nanboku-cho period and was also called Obi-in. In 1458 the Shimazu placed Shinnō Tadanori there, and the Ito and Shimazu fought over it for more than a century until Ito Suketaka took it in 1588. Three major earthquakes in the early Edo period led to repairs finished in 1691 and a palace completed in 1693.

The castle town below the walls keeps the street plan and plot divisions of the Edo-period jōkamachi, and in 1977 it became the first Important Traditional Buildings Preservation District in the Kyushu-Okinawa region. Within it stand the former residence of the last lord, Ito Sukeyori (Yoshōkan), the clan school Shintokudō, Iōshi Shrine, and the birthplace of Komura Jutarō, who helped negotiate the peace ending the Russo-Japanese War.

The castle grounds lie on the Shirasu plateau above the Sakatani River, reached by paved paths and steps from the Obi castle town. The main enclosure now holds Nichinan City Obi Elementary School, so parts of the site are not open to visitors. The 1978 gate and the wooden history museum stand near the entrance.

The 1978 main gate and the wooden history museum inside the grounds are reconstructions based on scholarly research, not on surviving depictions of the originals, so their appearance is an informed conjecture rather than a faithful restoration. The castle was chosen as No. 96 of Japan's 100 Famous Castles in 2006, and the NHK television novel Wakaba was filmed here.