The Battle of Miyako Bay (宮古湾海戦, Miyako-wan Kaisen) was a naval action on 6 May 1869, in which samurai loyalists of the former Tokugawa shogunate under the flag of the newly formed Republic of Ezo failed to take over the Kōtetsu, the flagship of the Imperial forces of the new Meiji government.

Despite the surrender of Edo Castle to the new Meiji government and heavy losses at the Battles of Ueno and Aizu, many of the military forces and leaders loyal to the former Tokugawa shogunate refused to accept defeat.

With the Ōuetsu Reppan Dōmei in tatters, a portion of the Tokugawa Navy led by Admiral Enomoto Takeaki fled to the northern island of Hokkaidō, together with several thousand soldiers and a handful of French military advisors, and established the Republic of Ezo.

Other ships included Kasuga, Hiryū, Dai Ichi Teibō, Yōshun, and Mōshun, which had been supplied by the domains of Saga, Chōshū and Satsuma to the new central government in 1868.

Anticipating that the Imperial fleet was en route to invade Ezo, Enomoto Takeaki decided to dispatch his three warships under the command of Kaigun bugyo Arai Ikunosuke in a boarding operation to seize the revolutionary new warship Kōtetsu, which would severely cripple the Imperial forces and would buy time for the Republic of Ezo to prepare for invasion or to negotiate more favourable terms with the Satchō Alliance.

Most of the attacking samurai perished; Nicol was hit by two bullets, and boarding party commander Kōga Gengo [ja] was killed and his position taken over by Admiral Arai Ikunosuke.