Dewa was born as the son of a samurai of the Aizu domain. As a youth, he enlisted in the Byakkotai, a reserve unit of the Aizu domain's official military. The Byakkotai was called into action, and Dewa served at the Battle of Aizu in the Boshin War. Dewa attended the 5th class of the Imperial Japanese Naval Academy, graduating 6th out of 43 cadets. He was appointed a midshipman on 16 August 1878, promoted to ensign on 12 August 1880 and promoted to sub-lieutenant on 27 February 1883. He served as a junior officer on several vessels of the early Japanese Navy, including the corvette, ironclad warship, sloop Hōshō, ironclad warship, corvette, and cruisers, and. He was promoted to lieutenant on 7 April 1886 and to lieutenant-commander on 16 October 1890. From 1886-1890, he was executive officer on the cruiser. From 1893-1893, he was captain of the gunboats, and and was promoted in rank to captain on 7 December 1894. During the First Sino-Japanese War, Dewa served as a staff officer of the "Western Seas Fleet", a defensive force patrolling home waters. In 1893, he became Director of the Personnel Section in the Navy Ministry. He later captained the cruiser in 1898. Promoted to rear admiral on 20 May 1900, and vice admiral on 6 June 1904, during the Russo-Japanese War he was a commander of the 3rd Cruiser Squadron of the IJN 1st Fleet and took part in the naval Battle of Port Arthur, and the Battle of the Yellow Sea,. During the decisive Battle of Tsushima he led the Third Squadron from his flagship, the cruiser. In December 1905, he was appointed Commander-in-Chief of the IJN 3rd Fleet, and from November 1906 was Director of the Naval Education Bureau. On 3 December 1907, Dewa was elevated to the peerage with the title of danshaku under the kazoku system. Later, he was successively Commander-in-Chief of the IJN 2nd Fleet, Sasebo Naval District, and the IJN 1st Fleet. On 9 July 1912, he was promoted to full admiral. Dewa Shigeto was the first non-Satsuma person to attain the rank of full admiral in the Imperial Japanese Navy. The first person of Aizu background to attain the rank of admiral was Hidematsu Tsunoda, and Matsudaira Morio the son of the former Aizu lord Matsudaira Katamori as a rear admiral. On the occasion of the Siemens-Vickers Navy Armament Scandal, as Chairman of the Commission of Inquiry he concentrated his efforts on the cleanup of corruption from the Navy. This eventually led to the fall of Admiral Yamamoto Gonnohyōe's cabinet in March 1914. Dewa retired from active service in 1925. In his later years, Dewa was involved with the construction of memorials to the casualties of the Battle of Aizu. His grave is at the Aoyama Cemetery in Tokyo.
