After the Russo-Japanese War began, the 8th Division was mobilized in June 1904. It was initially earmarked for a projected Japanese invasion of Primorskaya Oblast on the Siberian mainland, but was sent as a reserve force for the Siege of Port Arthur instead after that plan was shelved. It was then earmarked for the proposed invasion of Sakhalin, but the 13th Division was sent instead, The 8th Division remained in reserve until assigned as reinforcements at Battle of Sandepu in January 1905. It acted with distinction, repelling a Russian counterattack together with the 5th Division. In February 1905, the division participated in Battle of Mukden.