Yan Xishan or Yen Hsi-shan was a Chinese warlord who served in the government of the Republic of China. He effectively controlled the province of Shanxi from the 1911 Xinhai Revolution to the 1949 Communist victory in the Chinese Civil War. As the leader of a relatively small, poor, remote province, he survived Yuan Shikai, the Warlord Era, the Nationalist Era, the Japanese invasion of China and the subsequent civil war, being forced from office only when the Nationalist armies with which he was aligned had completely lost control of the Chinese mainland, isolating Shanxi from any source of economic or military supply. He has been viewed by Western biographers as a transitional figure who advocated using Western technology to protect Chinese traditions, while at the same time reforming older political, social and economic conditions in a way that paved the way for the radical changes that would occur after his rule.

A warlord is an individual who exercises military, economic, and political control over a region, often within a country without a strong national government, through usually informal or illegal coercive control over the local armed forces. Warlords have existed throughout much of history, albeit in a variety of different capacities within the political, economic, and social structure of states or ungoverned territories. The term is often applied in the context of China around the end of the Qing Dynasty, especially during the Warlord Era. The term can also be used for any supreme military leader.