Following his graduation from the Naval Staff College, Yashiro was promoted to captain in 1901. Commander of the armored cruiser during the Russo-Japanese War, Yashiro earned a reputation as a cool-headed and intelligent battle commander in skirmishes against the Russian cruiser and the gunboat in the Battle of Chemulpo Bay on 9 February 1904. Yashiro developed the plan to blockade the entrance to Port Arhur by sinking obsolete ships: however, he was overruled in his desire to lead the attack in person by Admiral Tōgō Heihachirō. The intimately unsuccessful operation was led by Takeo Hirose, and Yashiro was so overcome with remorse that he later personally sponsored the education of any children who had been orphaned in the operation. During the war, also participated in the Battle of the Yellow Sea and the Battle of Tsushima.