Khanasir (Arabic: خناصر / ALA-LC: Khanāṣir), also spelt Khanaseer, is a town located in Syria's as-Safira District. It is one of twenty-four towns and villages located in the Khanasir valley, an area with a population of 11,000 people.

The town is mentioned by Pliny the Elder as "Chenneseri"; the etymology of Khanasir does not exist in Aramaic, and an Arabic etymology is unlikely (an implausible one being "Khinsar" which mean "the little finger"). An Akkadian etymology is also possible, with the name deriving from the word "Hunsiru", a variant of the word "Humsiru" meaning a rat or a mouse.

During the Byzantine era, the town was known as Anasartha, enclosed within ramparts in western Syria; Malalas records that it was a kastron (fortified hilltop settlement) that was designated a polis by the Byzantine emperor Justinian I. A qanat dating back to Byzantine times that served as the water source for the village remained operational well into the 20th century. According to Robert L. France, Byzantine-era remains of Anasartha "are visible on the street, in newly built walls, and inside residential houses," in Khanasir today. Anasartha and its surrounding villages enjoyed a period of prosperity between the late 4th century and early 6th century. The vast majority of the houses and churches discovered in this region are of that period. The church in Anasartha itself dates from 426. Its bishop Maras took part in the Council of Chalcedon in 451, and his successor Cyrus was a signatory of the letter that the bishops of the province sent in 458 to the Byzantine Emperor Leo I the Thracian to protest about the killing of Proterius of Alexandria. Another bishop built a "refuge" in neighbouring Buz al-Khanzir in 506-507. No longer a residential bishopric, Anasartha is today listed by the Catholic Church as a titular see.

Khanasir, known to the early Arabs as "Khunasira", became a fortified estate and frequent residence of the Umayyad caliph Umar II (r. 717–720). He died and was buried there.

At the turn of the 20th century, Circassian immigrants from Manbij, northeast of Aleppo, settled in Khanasir, using old building materials from the site to reestablish the town. While the Byzantine-era qanat ceased to supply water to the village after the construction of pump wells in the area west of the Khanasir valley in 1975, the 12.0 km (7.5 mi)-long structure was described by Hamidé in 1959 as discharging 8 litres per second, irrigating a land area of 0.15 km² (37 acres).

On 23 February 2016, the Syrian Observatory for Human Rights reported that the Islamic State had captured the town which is located along a major road and supply route to the city of Aleppo. Two days later, the Syrian Arab Army, backed by Russian airstrikes and Hezbollah fighters, managed to recapture the town back from Islamic State militants.

Anasartha is the site of a number of Christian inscriptions into stone, or epigraphs.

Geoffrey Greatrex [de] and Samuel N. C. Lieu write that building work continued in Anasartha in the seventh century and that these epigraphs provide evidence of Roman resistance to Persian invasions.