Orhan Gazi (Ottoman Turkish: اورخان غازی، اورخان بن عثمان بن ارطغرل‎; Turkish: Orhan Gazi; 1281 – March 1362) was the second bey of the nascent Ottoman Sultanate (then known as the Ottoman Beylik or Emirate) from 1323/4 to 1362. He was born in Söğüt, as the son of Osman Gazi and Malhun Hatun. His grandfather was Ertuğrul, and his grandmother was Halime Hatun.

In the early stages of his reign, Orhan focused his energies on conquering most of northwestern Anatolia. The majority of these areas were under Byzantine rule and he won his first battle, at Pelekanon, against the Byzantine Emperor Andronikos III Palaiologos. Orhan also occupied the lands of the Karasids of Balıkesir and the Ahis of Ankara.

A series of civil wars surrounding the ascension of the nine-year-old emperor John V Palaiologos benefited Orhan greatly. In the Byzantine civil war of 1341–1347, the regent John VI Kantakouzenos married his daughter Theodora to Orhan and employed Ottoman warriors against the rival forces of the empress dowager, allowing them to loot Thrace. In the Byzantine civil war of 1352–1357, Kantakouzenos used Ottoman forces against John V himself, granting them the use of a European fortress at Çimpe around 1352. A major earthquake devastated Gallipoli (modern Gelibolu) two years later and Orhan's son Süleyman Pasha occupied the town, giving the Ottomans a strong bridgehead into mainland Europe.