Prince Yamagata Aritomo (山縣 有朋, 14 June 1838 – 1 February 1922) was a Japanese politician and general who served as prime minister of Japan from 1889 to 1891, and from 1898 to 1900. He was also a leading member of the genrō, a group of senior courtiers and statesmen who dominated the politics of Japan during the Meiji era. As the Imperial Japanese Army's inaugural Chief of Staff, he was the chief architect of its nationalist and reactionary ideology.

Yamagata Aritomo was born in the Chōshū Domain to a low-ranking samurai family, and after the opening of Japan in 1854 became active in the movement to overthrow the shogunate. As a member of the new government after the Meiji Restoration of 1868, he went overseas to study military systems, and from 1873 headed the Army Ministry. Yamagata was instrumental in drafting the Conscription Ordinance of 1873 and quelling the Satsuma Rebellion of 1877. He also was involved in the Imperial Rescript to Soldiers and Sailors of 1882 and the Imperial Rescript on Education of 1890. In 1900, he enacted a law permitting only generals and admirals on active duty to hold a cabinet post as army or navy minister, which gave the military control over the formation of future cabinets. Yamagata held senior military positions in the First Sino-Japanese War and Russo-Japanese War, achieving the rank of field marshal (Gensui) and later the title of prince in 1907.

From 1900 onward, Yamagata vied against Itō Hirobumi for supremacy among the Meiji oligarchy. Following Ito's assassination in 1909, he emerged as the most powerful genrō within the imperial court. However, a political scandal involving his meddling in Crown Prince Hirohito's engagement led to him losing power shortly before his death in 1922.