Ion Antonescu (; Romanian: [i'on antoˈnesku] (listen); 14 June [O.S. 2 June] 1882 – 1 June 1946) was a Romanian military officer and marshal who presided over two successive wartime dictatorships as Prime Minister and Conducător during most of World War II.

A Romanian Army career officer who made his name during the 1907 peasants' revolt and the World War I Romanian Campaign, the antisemitic Antonescu sympathized with the far-right and fascist National Christian and Iron Guard groups for much of the interwar period. He was a military attaché to France and later Chief of the General Staff, briefly serving as Defense Minister in the National Christian cabinet of Octavian Goga as well as the subsequent First Cristea cabinet, in which he also served as Air and Marine Minister. During the late 1930s, his political stance brought him into conflict with King Carol II and led to his detainment. Antonescu nevertheless rose to political prominence during the political crisis of 1940, and established the National Legionary State, an uneasy partnership with the Iron Guard's leader Horia Sima. After entering Romania into an alliance with Nazi Germany and ensuring Adolf Hitler's confidence, he eliminated the Guard during the Legionary Rebellion of 1941. In addition to being Prime Minister, he served as his own Foreign Minister and Defense Minister. Soon after Romania joined the Axis in Operation Barbarossa, recovering Bessarabia and Northern Bukovina, Antonescu also became Marshal of Romania.

An atypical figure among Holocaust perpetrators, Antonescu enforced policies independently responsible for the deaths of as many as 400,000 people, most of them Bessarabian, Ukrainian and Romanian Jews, as well as Romanian Romani. The regime's complicity in the Holocaust combined pogroms and mass murders such as the Odessa massacre with ethnic cleansing, and systematic deportations to occupied Transnistria. The system in place was nevertheless characterized by singular inconsistencies, prioritizing plunder over killing, showing leniency toward most Jews in the Old Kingdom, and ultimately refusing to adopt the Final Solution as applied throughout Nazi-occupied Europe. This was made possible by the fact that Romania, as a junior ally of Nazi Germany, was able to avoid being occupied by the Wehrmacht and preserve a degree of political autonomy.

Aerial attacks on Romania by the Allies occurred in 1944 and Romanian troops suffered heavy casualties on the Eastern Front, prompting Antonescu to open peace negotiations with the Allies, ending with inconclusive results. On 23 August 1944, the king Michael I led a coup d'état against Antonescu, who was arrested; after the war he was convicted of war crimes, and executed in June 1946. His involvement in the Holocaust was officially reasserted and condemned following the 2003 Wiesel Commission report.