Masada (Hebrew: מְצָדָה məṣādā, 'fortress'; Arabic: جبل مسعدة, romanized: jabal musayda, lit. 'mount museida') is a mountain-top fortress complex in the Judaean Desert, overlooking the western shore of the Dead Sea in southeastern Israel. The fort, built in the first century BC, was constructed atop a natural plateau rising over 400 m (1,300 ft) above the surrounding terrain, 20 km (12 mi) east of modern Arad.

The most significant remains at the site date to the reign of Herod the Great, King of Judaea c. 37–4 BC, who transformed Masada into a fortified desert refuge early in his rule. He enclosed the summit with a casemate wall and towers, and constructed storerooms, an advanced water system, and bathhouses, along with two elaborate palaces: one on the western side and another built across three terraces on the northern cliff. These palaces remain among the finest examples of Herodian architecture.

Masada is most renowned for its role during the First Jewish–Roman War (66–73 AD), when it became the final holdout of Jewish rebels following the destruction of Jerusalem. A group known as the Sicarii, a radical faction led by Eleazar ben Ya'ir, defended the site against the Roman Tenth Legion under Lucius Flavius Silva. The Romans laid siege by building a circumvallation wall and a massive ramp. According to Josephus, when the walls were breached in 73/74 AD, the Romans found nearly 1,000 inhabitants had died by mass suicide—a claim that remains debated among historians. In modern times, the story of Masada was interpreted as a symbol of heroism that became influential in early Israeli national identity.

Excavations led by archaeologist Yigael Yadin in the 1960s uncovered remarkably preserved remains, including Herod's palaces, storerooms with food remnants, ritual baths, a synagogue, Jewish scrolls, columbaria, and pottery shards bearing names, one inscribed "ben Ya'ir," possibly linked to the final days of the defenders, and a small Byzantine church. The surrounding Roman siege works and bases remain visible and are among the most intact examples of Roman military engineering. Today, Masada is a UNESCO World Heritage Site due to those siege works, and one of Israel's most popular tourist attractions, drawing around 750,000 visitors a year.

The cliff of Masada is, geologically speaking, a horst. As the plateau abruptly ends in cliffs steeply falling about 400 m (1,300 ft) to the east and about 90 m (300 ft) to the west, the natural approaches to the fortress are very difficult to navigate. The top of the mesa-like plateau is flat and rhomboid-shaped, about 550 m (1,800 ft) by 270 m (890 ft). Herod built a 4 m (13 ft) high casemate wall around the plateau totaling 1,300 m (4,300 ft) in length, reinforced by many towers. The fortress contained storehouses, barracks, an armory, a palace, and a series of cisterns (capacity around 40,000 m³; 1.4 million cu ft) that were refilled by rainwater – with the runoff collected from a single day's rain allegedly able to sustain over 1,000 people for 2 to 3 years. Three narrow, winding paths led from below up to fortified gates.

Almost all historical information about Masada comes from the first-century Jewish Roman historian Josephus. Masada is also mentioned in the Judean Desert Documents.

Josephus writes that the site was first fortified by Hasmonean ruler Alexander Jannaeus in the first century BC. However, so far no Hasmonean-period building remains could be identified during archaeological excavations.

Josephus further writes that Herod the Great captured it in the power struggle that followed the death of his father Antipater in 43 BC. It survived the siege of the last Hasmonean king Antigonus II Mattathias, who ruled with Parthian support.