The March–April 1605 papal conclave was convened on the death of Pope Clement VIII and ended with the election of Cardinal Alessandro Ottaviano de' Medici as Pope Leo XI. It was the first of two papal conclaves in 1605; Leo died on 27 April 1605, twenty-six days after he was elected. The conclave was dominated by conflict over whether Cesare Baronius should be elected pope, and Philip III of Spain excluded both Baronius and the eventually successful candidate, Medici.

Pope Leo XI, born Alessandro Ottaviano de' Medici, was head of the Catholic Church and ruler of the Papal States from 1 April 1605 to his death, on 27 April 1605. His pontificate is one of the briefest in history, having lasted under a month. He was from the prominent House of Medici originating from Florence. Medici's mother opposed his entering the priesthood and sought to prevent it by having him given secular honours, but after her death he eventually was ordained a priest in 1567. In his career he served as Florence's ambassador to the pope, Bishop of Pistoia, Archbishop of Florence, Papal legate to France, and as the cardinal Prefect for the Congregation of Bishops and Regulars. He was elected to the papacy in the March 1605 papal conclave and served as pope for 27 days.