Viscount Niwa Nagakuni (丹羽長国, May 22, 1834 – January 15, 1904) was an Edo period Japanese samurai, and the 10th daimyō of Nihonmatsu Domain in the Tōhoku region of Japan.

These expenses were compounded by the economic difficulties the domain faced following the Tenpō famines, as well as bureaucratic corruption.

In 1868, with the start of the Boshin War, Nihonmatsu joined the Ōuetsu Reppan Dōmei, but was defeated by the forces of the Satchō Alliance in every engagement.

He subsequently made peace with the Meiji government and was ordered to relocated to Tokyo, where he was held under house arrest at the residence of Maebashi Domain.

His adopted son Niwa Nagahiro (brother of Uesugi Mochinori of Yonezawa Domain) succeeded him.