Sa'ir (Arabic: سعير, also spelled Saeer, Seir, or Si'ir) is a Palestinian town in the Hebron Governorate of the State of Palestine, in the southern West Bank, located 8 kilometers (5.0 mi) northeast of Hebron. Nearby localities include Beit Fajjar and al-Arroub to the north, Beit Ummar to the northwest, Halhul to the west and Beit Einun and ash-Shuyukh to the south. The Dead Sea is just east of Sa'ir's municipal borders. In the 2017 census Palestinian Central Bureau of Statistics, the town had a population of 20,722.

It has municipal jurisdiction over 117,000 dunams, 6,000 of which is built-up area and 11,715 of which is cultivated. The main economic activities in Sa'ir are agriculture and the Israeli labor market, although the latter has been adversely affected as a result of the Israeli restrictions following the Second Intifada in 2000–04. Olives are the major cash crop.

It is often thought that Si'ir is identical with Zior (or Zi'or), a biblical town mentioned in the Book of Joshua (15:54) among the cities of Judah, near Hebron and others in the Hebron Hills. Yet, not all scholars accepted this identification. According to ARIJ, Sa'ir "was established on the town of Saeer (صعير) or Saiour (صعيور)", and during the Roman era the town was known as "Sior".

An archaeological survey of the village revealed pottery from the Iron Age II (under the Kingdom of Judah), Persian, Roman, Byzantine (the predominant finds), medieval, and Ottoman periods. The village's dense core, located near the spring, is the site of the ancient settlement's tell. Nearby caves yielded remains from the Chalcolithic and Early Bronze Age periods. In 2024, Talmudic-era menorah engravings were discovered on a doorway slab in the village's kasbah.

The PEF's Survey of Western Palestine (SWP), wrote that: "The tomb of El 'Ais (Esau), south of the village, is in a chamber 37 feet east and west by 20 feet north and south, th a Mihrab on the south wall. The tomb is 12 feet long, 3 1/2 feet broad, 5 feet high, covered with a dark green cloth and a canopy above. An ostrich egg is hung near. North of the chamber is a vaulted room of equal size, and to the east is an open court with a fig-tree, and a second cenotaph rudely plastered, said to be that of Esau's slave. Rock-cut tombs exist south-west of this place."

A heart-shaped lamp marked with arches framing birds, today on display at the Flagellation Museum, is marked as coming from Sa'ir. Similar lamps are dated to the early Islamic period.

Suriano, a 15th century Franciscan custos, wrote that the House of Isaac, then a mosque where Muslims worship, can be found in Syeir. He mentioned that he had seen Jewish antiquities there and described the area as not being sown but rather untamed, arid, and a habitat for animals and game.

In 1596 Sa'ir appeared in the Ottoman tax registers as part of the nahiya of Halil in the Liwa of Quds. It had an entirely Muslim population consisting of 72 households. Taxes were paid on wheat, barley, summer crops, olive trees, goats and/or beehives.