Tatsuoka Castle (龍岡城, Tatsuoka-jō) was a Bakumatsu period pentagonal "star fort" located in what is now part of the city of Saku, Nagano prefecture.

Along with the Goryōkaku in Hakodate, Hokkaidō, it is one of only two star fortresses in Japan, and has been the castle ruins have been designated as a National Historic Site since 1934.

Tatsuoka Castle was built by Matsudaira Norikata (1829-1910) of the Ogyū-Matsudaira clan at the end of the Edo period.

[2] Following the Perry Expedition and the arrival of the kurofune threatening Japan's national isolation policy, Matsudaira Norikata was appointed a Obangashira, or commander of Shogunate troops in 1863, only at 24 years old.

The castle site is about 20 to 30 minutes on foot from Usuda Station on the JR East Koumi Line; however, it is not open to the public.