Suleiman I (Ottoman Turkish: سلیمان اول, romanized: Süleymân-ı Evvel; Modern Turkish: I. Süleyman, IPA: [biɾinˈdʒi sylejˈman]; 6 November 1494 – 6 September 1566), commonly known as Suleiman the Magnificent in the Western world and as Suleiman the Lawgiver (قانونی سلطان سلیمان, Ḳânûnî Sulṭân Süleymân) in his own realm, was the Ottoman sultan between 1520 and his death in 1566. Under his administration, the Ottoman Empire ruled over at least 25 million people.

After succeeding his father Selim I on 30 September 1520, Suleiman began his reign by launching military campaigns against the Christian powers of Central and Eastern Europe and the Mediterranean; Belgrade fell to him in 1521 and Rhodes in 1522–1523, and at Mohács in 1526, Suleiman broke the strength of the Kingdom of Hungary. Hungary was subsequently divided, with much of it incorporated directly into the empire. However, his defeat at the siege of Vienna in 1529 checked advances further into Europe.

Presiding over the apex of the Ottoman Empire's economic, military, and political strength, Suleiman rose to become a prominent monarch of the 16th century, as he personally led Ottoman armies in their conquests of a number of European Christian strongholds. He also fought for years against the Shia Muslim Safavid Empire of Persia, resulting in the annexation of Mesopotamia. Ottoman Tripolitania was established in North Africa. The Ottoman fleet dominated the seas from the Mediterranean to the Red Sea and through the Persian Gulf.

At the helm of the rapidly expanding Ottoman Empire, Suleiman personally instituted major judicial changes relating to society, education, taxation, and criminal law. His reforms, carried out in conjunction with the Ottoman chief judicial official Ebussuud Efendi, harmonized the relationship between the two forms of Ottoman law: sultanic (Kanun) and Islamic (Sharia). He was a distinguished poet and goldsmith; he also became a great patron of fine culture, overseeing the "Golden Age" of the Ottoman Empire in its artistic, literary, and architectural development.

In 1533, Suleiman broke with Ottoman tradition by marrying Roxelana (Ukrainian: Роксолана), a woman from his Imperial Harem. Roxelana, so named in Western Europe for her red hair, was a Ruthenian who converted to Sunni Islam from Eastern Orthodox Christianity and thereafter became one of the most influential figures of the "Sultanate of Women" period in the Ottoman Empire. Upon Suleiman's death in 1566, which ended his 46-year-long reign, he was succeeded by his and Roxelana's son Selim II. Suleiman's other potential heirs, Mehmed and Mustafa, had died; Mehmed had succumbed to smallpox in 1543, while Mustafa had been executed via strangling on Suleiman's orders in 1553. His other son Bayezid was also executed on his orders, along with Bayezid's four sons, after a rebellion in 1561. Although scholars typically regarded the period after his death to be one of crisis and adaptation rather than of simple decline, the end of Suleiman's reign was a watershed in Ottoman history. In the decades after Suleiman, the Ottoman Empire began to experience significant political, institutional, and economic changes—a phenomenon often referred to as the Era of Transformation.

Suleiman the Magnificent (محتشم سلیمان Muḥteşem Süleymân), as he was known in the West, was also called Suleiman the First (سلطان سلیمان اول Sulṭān Süleymān-ı Evvel), and Suleiman the Lawgiver (قانونی سلطان سلیمان Ḳânûnî Sulṭân Süleymân) for his reform of the Ottoman legal system.

It is unclear when exactly the term Kanunî (the Lawgiver) first came to be used as an epithet for Suleiman. It is entirely absent from sixteenth and seventeenth-century Ottoman sources and may date from the early 18th century.

There is a tradition of Western origin, according to which Suleiman the Magnificent was "Suleiman II", but that tradition has been based on an erroneous assumption that Süleyman Çelebi was to be recognised as a legitimate sultan.