Mustafa I (June 24, 1591 – January 20, 1639) (Ottoman Turkish: مصطفى اول‎), called Mustafa the Saint (Veli Mustafa) during his second reign and often called Mustafa the Mad (Deli Mustafa) by modern historians, was the son of Mehmed III and was the Sultan of the Ottoman Empire from 1617 to 1618 and from 1622 to 1623.

He was born in the Manisa Palace, as the younger brother of Ahmed I (1603–17). His mother was Halime Sultan, an Abkhazian lady.

Before 1603 it was customary for an Ottoman Sultan to have his brothers executed shortly after he gained the throne (Mustafa's father Mehmed III had executed 19 of his own brothers). But when the thirteen-year-old Ahmed I was enthroned in 1603, he spared the life of the twelve-year-old Mustafa.

Mustafa might have been left alive because Ahmed had not yet produced any sons, so at the time Mustafa was his only heir. Though Ahmed went on to father several sons, he did not execute Mustafa, perhaps because of his brother's apparent mental problems. Another factor in Mustafa's survival is the influence of Kösem Sultan (Ahmed's favorite consort), who may have wished to preempt the succession of Osman, Ahmed’s first-born son from another concubine. However, the reports of foreign ambassadors suggest that Ahmed actually liked his brother. If Osman became Sultan, he would likely try to execute his half-brothers, the sons of Ahmed and Kösem. This scenario later became a reality when Osman II executed his brother Mehmed in 1621.

Until Ahmed's death in 1617, Mustafa lived in the Old palace, along with his mother, and grandmother Safiye Sultan.

Ahmed's death created a dilemma never before experienced by the Ottoman Empire. Multiple princes were now eligible for the Sultanate, and all of them lived in Topkapı Palace. A court faction headed by the Şeyhülislam Esad Efendi and Sofu Mehmed Pasha (who represented the Grand Vizier when he was away from Istanbul) decided to enthrone Mustafa instead of Ahmed's son Osman. Sofu Mehmed argued that Osman was too young to be enthroned without causing adverse comment among the populace. The Chief Black Eunuch Mustafa Agha objected, citing Mustafa's mental problems, but he was overruled. Mustafa's rise created a new succession principle of seniority that would last until the end of the Empire. It was the first time an Ottoman Sultan was succeeded by his brother instead of his son.

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