Piri Reis (Turkish: Pîrî Reis; born Muhiddin Piri; c. 1470–1553) was an Ottoman Turkish cartographer, admiral, navigator, and corsair. He is best known for his 1513 world map and his nautical atlas, the Kitab-ı Bahriye (Book of the Sea). His maps combined classical sources, firsthand seafaring experience, and new European discoveries, engaging more directly with the Age of Discovery than other Ottoman works from the period.

Piri Reis began his maritime career sailing with his uncle, the corsair Kemal Reis, with whom he entered Ottoman naval service. He later commanded his own ship in the Ottoman–Venetian wars and after his uncle's death, began the cartographic work for which he became best known. After taking part in the 1517 conquest of Egypt, Piri Reis presented his world map and later his atlases as gifts to the Ottoman Sultans. After promotion to grand admiral of the Ottoman fleet in the Indian Ocean, he led successful campaigns in the Red Sea, but was executed following his retreat from the siege of Hormuz Island at the mouth of the Persian Gulf.

During his lifetime, Piri Reis' cartography received little appreciation, but many copies of the Kitab-ı Bahriye were produced after his death. The 1929 rediscovery of his first world map, during renovations to the Topkapı Palace in Istanbul, increased interest in his work in part because it cites many contemporary Portuguese explorers and a now-lost map by Christopher Columbus. The rediscovery made his career a point of national pride for Turkey. Although the map has been the subject of fringe theories based on the disproven hypothesis that it depicted an ice-free Antarctica, studies have shown no significant similarities between its southern continent and Antarctica's coast beneath the ice. Nevertheless, this speculation has broadened popular interest in Piri Reis' cartography.

The only surviving primary sources covering Piri Reis' early life are his own cartographic works, in which he identifies himself as Muhiddin Piri, son of Hacı Mehmed. Piri Reis was likely born between 1465 and 1470 in Gelibolu, Turkey, also known as Gallipoli. This was a major naval base for the Ottoman Empire, a Muslim power in the Mediterranean spanning from Anatolia in Asia into Eastern Europe. Sixteenth-century Ottoman historian Ibn Kemal wrote that Gelibolu had a strong maritime tradition where the town's children were "rocked to sleep with the lullaby of the sea and of the ships day and night"—a culture that, according to Turkish historian Afet İnan, shaped Piri's upbringing.

His uncle Kemal Reis was a notable corsair, a type of state-sanctioned pirate, acting often along religious lines in the Mediterranean. Serving under Kemal, he later earned the rank Reis himself, equivalent to a captain in the Ottoman Navy.

Cartography scholar Ibrahim Yilmaz suggests Piri Reis' parents likely died around 1481, after which he began working aboard his uncle's ship. Years later, Piri Reis described how, shortly after he began sailing with his uncle, a storm nearly destroyed Kemal Reis' galley in a small stony harbor near Mount Athos in modern-day Greece. While they were sheltering from rough seas, Eastern Orthodox monks came to the water with ropes and tied the galley down until the storm passed.

Under Sultan Bayezid II's approval, Piri Reis sailed west with Kemal Reis and other Ottoman corsairs to fight Catholic forces and aid the Emirate of Granada, the last Muslim-ruled territory in Iberia. As a teenager, he helped his uncle bombard the forces besieging Málaga in 1487.

By the end of 1487, Málaga fell to the Spanish, but Barbary corsairs based in northern Africa and led by Kemal Reis continued to threaten European maritime traffic. Piri Reis later wrote, "We sailed on the Mediterranean and fought the enemies of our religion mercilessly." During the winters, he and his uncle took shelter in harbors along the Barbary Coast of North Africa, including Béjaïa in modern-day Algeria. For six years, they sailed north to raid the coasts of Spain, Corsica, Sardinia, and Sicily.