Sirte (/ˈsɜːrt/; Arabic: سِرْت, pronunciation^ⓘ), also spelled Sirt, Surt, Sert or Syrte, is a city in Libya. It is located south of the Gulf of Sirte, almost right in the middle between Tripoli and Benghazi. It is famously known for its battles, ethnic groups and loyalty to former Libyan ruler Muammar Gaddafi. Due to developments in the First Libyan Civil War, it was briefly the capital of Libya from 1 September to 20 October 2011 as Tripoli's successor after the city's fall. The settlement was established in the early 20th century by the Italians, at the site of a 19th-century fortress built by the Ottomans. It grew into a city after World War II.

Contrary to popular belief, Sirte was not Muammar Gaddafi's birthplace. Gaddafi's birthplace was in a village 20 km south of Sirte, which is called Qasr Abu Hadi. The inhabitants of this village were farmers. Just a few significant people from the Gaddafi tribe, of whom some were born in Sirte, were appointed to government roles during the time of the Libyan Arab Jamahiriya until the NATO-led invasion of Libya in 2011. Sirte was favoured by the Gaddafi government. The city was the final major stronghold of Gaddafi loyalists in the civil war and Gaddafi was killed there by rebel forces on 20 October 2011 after sustaining major injury caused by French Air Force Bombs discharged as part of the NATO intervention. During the battle, Sirte was left almost completely in ruins, with many buildings destroyed or damaged. Six months after the civil war, almost 60,000 inhabitants, more than 70 percent of the pre-war population, had returned.

Sirte is built near the site of the ancient Phoenician city of Macomedes-Euphranta, which was an important link on the road along the Mediterranean Sea littoral. It is the last confirmed place where the Punic language was spoken, in the 5th century CE. The region had no recognized administrative centre and was infested for centuries by bandits. In Classical times, the coast was "proverbially dangerous to shipping", called "inhospita Syrtis" in Virgil's Aeneid. John Milton's Paradise Lost Book 2 lines 939-940 speaks of "a boggy Syrtis, neither sea/Nor good dry land".

The medieval city of Surt was located some 55 km east of the present-day city, at a site now known as al-Mudayna or Madina Sultan. After the Umayyad conquest of North Africa, Berbers from the Butr confederation settled in Surt, and around the middle of the 8th century they converted to Ibadi Islam along with the surrounding region. A mosque was probably built at Surt during this period, although no authors mention a mosque in Surt until the 11th century. The most detailed early description of the city was written by Ibn Hawqal, who passed through Surt in 947 on his way to al-Mahdiyyah (which was then the capital of the Fatimid Caliphate). Ibn Hawqal described Surt as being "a bow-shot away from the sea, built on hard, sandy ground with strong walls of mud and brick". He described it as inhabited by Berbers, who stored rainwater in cisterns and were engaged in various forms of agriculture and livestock herding. They grew dates, grapes, and other fruits, and kept goats and camels. Another local industry was alum mining, which was exported. According to Ibn Hawqal, Surt at this point was wealthier than nearby Ajdabiya.

Surt was probably fortified by the Fatimid caliph al-Mu'izz around 965, in preparation for the Fatimid conquest of Egypt under his general Jawhar al-Siqilli. The Fatimids founded the new city of Cairo to serve as their new capital in Egypt. After they moved there, the Surt region became a battleground between the Fatimids and the Zirid dynasty of Kairouan. The Banu Khazrun of Tripoli also controlled Surt for a while as Fatimid allies. Around 1037, the Banu Hilal began to settle in the Surt region. Somewhat later, al-Bakri described Surt as "a large city by the sea" with a mosque, a hammam, and bazaars (these three features are mentioned for the first time here). He listed three gates in the city walls: Qiblī (facing southeast), Jawfī (facing inland), and "a small one facing the sea". There were no suburbs outside the walls. He also noted, "its animals are goats and their meat is juicy and tender, the like of which is not found in Egypt." Al-Bakri also alluded to a merchant community including Arabs, Berbers, Persians, and Copts.

In the late Fatimid period, Surt began to decline - it seems to have lost its position at the intersection of east-west and north-south trade routes. The 12th-century author al-Idrisi apparently visited the Surt region and wrote about the city's decline. The 13th-century author Ali ibn Sa'id al-Maghribi wrote that its forts were still standing. At some point thereafter, the old city of Surt was finally abandoned.

Since the 19th century and particularly since the 1960s, the old city of Surt has been explored by archaeologists, mostly based on al-Bakri's reports. Excavations have revealed the old city walls, enclosing an area of 184,003 square meters, as well as the gates, the forts, the mosque, and the city streets. No evidence of the harbor has been found, though.

In 1842 the Ottomans built a fortress at Marsat al Zaafran ("saffron harbour") which became known as Qasr al Zaafran ("saffron castle"), and later as Qasr Sert. The fortress was built under sultan Abdülmecid I as part of the restoration of Ottoman control over Tripolitania after the fall of the Karamanli dynasty. It was around this fortification, which was taken over and repaired by the Italians in 1912, that the settlement of Sirte grew up.