The Siwa Oasis (Arabic: واحة سيوة Wāḥat Sīwah [ˈwæːħet ˈsiːwæ]) is an urban oasis in Egypt. It is situated between the Qattara Depression and the Great Sand Sea in the Western Desert, 50 kilometres (31 mi) east of the Egypt–Libya border and 560 kilometres (350 mi) from the Egyptian capital city of Cairo. It is famed from its role in ancient Egypt as the home to an oracle of Amun, the ruins of which are a popular tourist attraction, giving it the ancient name Oasis of Amun-Ra, after the major Egyptian deity.

The oasis is in a deep depression that reaches −19 metres (−62 ft) below sea level. To the west, the al Jaghbub Oasis rests in a similar depression and to the east, the large Qattara Depression is also below sea level. The depression is fertile due to both natural flowing artesian wells and irrigation. It is the site of about 200 natural springs. Siwa is directly adjacent to the Libyan Desert plateau. The geology is characterised by horizontal layers of porous limestones alternated with marls and clays dating back to the Miocene. The limestone plateau and inselbergs resulting from the oasis' erosion along the dunes create reliefs that the Isiwan describe as mountains (adrar in Tamizight). Two large salt lakes are fed by drainage water of agricultural origin. The oasis supports the cultivation of thousands of date palms and olives. Siwa has a temperate desert climate.

The Ancient Egyptian name of the oasis was sḫt jꜣmw, meaning "Field of Trees". The native Libyan toponym may be preserved in the Egyptian t̠ꜣ(j) n d̠rw "tꜣj on the fringe" where t̠ꜣ transcribed the local Palaeo-Berber name *Se or *Sa. This name survived in the works of Muslim geographers as سنترية Santariyyah.

Siwah is the Arabic name of the oasis called Sali in Berber. The oasis is also called Isiwan in modern Berber.

The etymology of the word سيوة Siwah is unclear. Champollion derives it from Coptic ⲥⲟⲟⲩϩ (soouh) – a corruption of Egyptian word for "oasis", ⲟⲩⲁϩ (ouoh). The additional evidence of the Egyptian source of Siwa's name is another place name in Kharga Oasis that may share the same etymology – S.t-wȝḥ, modern Deir el-Hagar). Basset links it to a Berber tribal name swh attested further west in the early Islamic period, while Ilahiane, following Chafik, links it to the Shilha Berber word asiwan, a type of bird of prey, and hence to Amun-Ra, one of whose symbols was the falcon. Some classical authors referred to the site as "Ammonium".

Although the site is known to have been settled since at least the 10th millennium BCE, the earliest evidence of any connection with Ancient Egypt is the 26th Dynasty, when a necropolis was established. Ancient Greek settlers at Cyrene made contact with the oasis around the same time (7th century BCE), and the oracle temple of Amun (Greek: Zeus Ammon), who, Herodotus was told, took the image here of a ram. Herodotus knew of a "fountain of the Sun" that ran coldest in the noontime heat. During his campaign to conquer the Persian Empire, Alexander the Great reached the oasis, supposedly by following birds across the desert. The oracle, Alexander's court historians alleged, confirmed him as both a divine personage and the legitimate Pharaoh of Egypt, though Alexander's motives in making the excursion, following his founding of Alexandria, remain to some extent inscrutable and contested. During the Ptolemaic Kingdom, its Ancient Egyptian name was sḫ.t-ỉm3w, meaning "Field of Trees".

In the 12th century, Al-Idrisi mentions it as being inhabited mainly by Berbers, with an Arab minority; a century before Al-Bakri stated that only Berbers lived there. The Egyptian historian Al-Maqrizi traveled to Siwa in the 15th century and described how the language spoken there 'is similar to the language of the Zenata'.

The first European to document a visit since Roman times was the English traveler William George Browne, who came in 1792 to see the ancient temple of the Oracle of Amun. Bompiani, in her description of the 19th-century explorer Luigi Robecchi Bricchetti, called this site the Oasis of Jupiter Ammon.