Nusaybin (pronounced [nuˈsajbin], Kurdish: Nisêbîn) is a municipality and district of Mardin Province, Turkey. Its area is 1,079 km², and its population is 115,586 (2022). The city is populated by Kurds of different tribal affiliation.

Nusaybin is separated from the larger Kurdish-majority city of Qamishli by the Syria–Turkey border.

The city is at the foot of the Mount Izla escarpment at the southern edge of the Tur Abdin hills, standing on the banks of the Jaghjagh River (Turkish: Çağçağ), the ancient Mygdonius (Ancient Greek: Μυγδόνιος). The city existed in the Assyrian Empire and is recorded in Akkadian inscriptions as Naṣibīna. Having been part of the Achaemenid Empire, in the Hellenistic period the settlement was re-founded as a polis named "Antioch on the Mygdonius" by the Seleucid dynasty after the conquests of Alexander the Great. A part of first the Roman Republic and then the Roman Empire, the city (Latin: Nisibis; Ancient Greek: Νίσιβις) was mainly Syriac-speaking, and control of it was contested between the Kingdom of Armenia, the Romans, and the Parthian Empire. After a peace treaty contracted between the Sasanian Empire and the Romans in 298 and enduring until 337, Nisibis was capital of Roman Mesopotamia and the seat of its governor (Latin: dux mesopotamiae). Jacob of Nisibis, the city's first known bishop, constructed its first cathedral between 313 and 320. Nisibis was a focus of international trade, and according to the Greek history of Peter the Patrician, the primary point of contact between Roman and Persian empires.

Nisibis was besieged three times by the Sasanian army under Shapur II (r. 309–379) in the first half of the 4th century; each time, the city's fortifications held. The Roman soldier and Latin historian Ammianus Marcellinus described Nisibis, fortified with walls, towers, and a citadel, as "the strongest bulwark of the Orient". After the defeat of the Romans in Julian's Persian War, Julian's successor Jovian (r. 363–364) was forced to cede the five Transtigritine provinces to the Persians, including Nisibis. The city was evacuated and its citizens forced to migrate to Amida (Diyarbakır) – which was expanded to accommodate them – and to Edessa (Urfa). Nisibis remained a major entrepôt; one of only three such cities of commercial exchange allowed by Roman law promulgated in 408/9. However, despite several Roman attempts to recapture Nisibis through the remainder of the Roman–Persian Wars and the construction of nearby Dara to defend against Persian attack, Nisibis was not returned to Roman control before it was conquered in 639 by the Rashidun Caliphate during the Muslim conquest of the Levant.

Under Sasanian rule and after, Nisibis was a major centre of the Christian Church, and the bishop of Nisibis attended the Council of Seleucia-Ctesiphon convened in 410 by the emperor Yazdegerd I (r. 399–420). As a result of this council, the Church of the East was established, and the bishop of Nisibis became the metropolitan bishop of the five erstwhile Transtigritine provinces. Narsai, formerly a theologian at the School of Edessa, founded the famous School of Nisibis with the bishop, Barsauma, in the 470s. When the Roman emperor Zeno (r. 474–491) closed the School of Edessa in 489, the scholars migrated to Nisibis's school and established the city as the foremost centre of Christian thought in the Church of the East. According to the Damascene monk John Moschus, the city's cathedral had five doors in the 7th century, and the monastic and later bishop of Harran, Simeon of the Olives, was recorded as having renewed several ecclesiastical buildings in the early period of Arab rule. The monasteries of the nearby Tur Abdin, led by the reforms of Abraham the Great of Kashkar, founder of the "Great Monastery" of Mount Izla, underwent substantial revival in the years after the Muslim conquest. However, besides the baptistery known as the Church of Saint Jacob (Mar Ya‘qub) and built in 359 by bishop Vologeses, little remains of ancient Nisibis, probably because of ruinous earthquake in 717. Archaeological excavations were conducted in the vicinity of the 4th-century baptistery in the early 21st century, revealing various buildings including the 4th-century cathedral.

First mentioned in 901 BCE, Naṣibīna was an Aramean kingdom captured by the Assyrian king Adad-Nirari II in 896. By 852 BCE, Naṣibīna had been fully annexed to the Neo-Assyrian Empire and appeared in the Assyrian Eponym List as the seat of an Assyrian provincial governor named Shamash-Abua.

It was under Babylonian control until 536 BCE, when it fell to the Achaemenid Persians, and remained so until taken by Alexander the Great in 332 BCE.

The Seleucids re-founded the city as Antiochia Mygdonia (Greek: Ἀντιόχεια τῆς Μυγδονίας), mentioned for the first time in Polybius' description of the march of Antiochus III the Great against Molon (Polybius, V, 51). The Greek historian Plutarch suggested that the city was populated by descendants of Spartans. Around the 1st century CE, Nisibis (Hebrew: נציבין, romanized: Netzivin) was the home of Judah ben Bethera, who founded a famous yeshiva there.