Mustafa III (Ottoman Turkish: مصطفى ثالث Muṣṭafā-yi sālis) (January 28/18, 1717 January 21, 1774) was the Sultan of the Ottoman Empire from 1757 to 1774. He was a son of Sultan Ahmed III (1703–30) and was succeeded by his brother Abdul Hamid I (1774–89). He was born in Edirne. His mother was Valide Sultan Amina Mihr-i Shah.

An energetic and perceptive ruler, Mustafa III sought to modernize the army and the internal state machinery to bring his empire in line with the Powers of Europe.

Unfortunately the Ottoman state had declined so far that any general attempts at modernization were but a drop in the ocean, while any major plans to change the administrative status quo immediately roused the conservative Janissaries and imams to the point of rebellion. Mustafa III did secure the services of foreign generals to initiate a reform of the infantry and artillery. The Sultan also ordered the founding of Academies for Mathematics, Navigation and the Sciences.

Well aware of his own military weakness, Mustafa III assiduously avoided war and was powerless to prevent the annexation of the Crimea by Catherine II of Russia (1762–96). However this action, combined with further Russian aggression in Poland compelled Mustafa III to declare war on Saint Petersburg shortly before his death.

He died at Topkapi Palace, Istanbul.

Mustafa married Valide Sultan Mihr-i shah (originally from Genoa), and had two sons: Selim, son of Mihr-i shah, and Mohammed. He also had five daughters.