Alchi Monastery (Tibetan: ཨ་ལྕི་ཆོས་འཁོར།) or Alchi Gompa (Tibetan: ཨ་ལྕི་དགོམ་པ།, also Alci) is a Tibetan Buddhist monastery, known more as a monastic complex (chos-'khor) of temples in Alchi village in the Leh District, under the Ladakh Autonomous Hill Development Council of the Union Territory of Ladakh, India. The complex comprises four separate settlements in the Alchi village in the lower Ladakh region with monuments dated to different periods. Of these four hamlets, Alchi monastery is said to be the oldest and most famous. It is administered by the Likir Monastery. It is 60 west of Leh on Leh-Kargil Highway.

Alchi is also part of the three villages (all in lower Ladakh region) which constitute the 'Alchi group of monuments'; the other two villages adjoining Alchi are the Mangyu and Sumda Chun. The monuments in these three villages are stated to be of "unique style and workmanship", but the Alchi monastic complex is the best known.

The Dukhang, or Assembly Hall and the Main Temple (gTsug-lag-khang), which is a three-storied temple called the Sumtseg (gSum-brtsegs, literally 'three-storied'), are built in the Kashmiri style as seen in many monasteries; the third temple is called the Manjushri Temple ('Jam-dpal lHa-khang). Chortens are also an important part of the complex.

The monastery complex was built, according to local tradition, by the great translator Guru Rinchen Zangpo between 958 and 1055. However, inscriptions in the preserved monuments ascribe the Sumtseg to a Tibetan noble called Kal-dan Shes-rab, and more recent translations of inscriptions in the Sumtseg and Dukhang suggest the Dukhang was built (and thus the complex founded) by an unnamed uncle of patron Tsültrim Ö about a generation prior, no earlier than 1140 CE.

The artistic and spiritual details of both Buddhist and the Hindu kings of the time in Kashmir and Himachal Pradesh are reflected in the wall paintings in the monastery. These are some of the oldest surviving paintings in Ladakh. The complex also has huge statues of the Buddha and elaborate wood carvings and art-work comparable to the baroque style. Shakti Maira has vividly explained the beauty of this small monastery.

The history of the monuments in the Alchi complex and in the other two villages in the Alchi group is not precise, in spite of many inscriptions and texts displayed on the walls. One inscription refers to the foundation of the monastery by a king and a queen, who may appear in the royal drinking scene from the murals at the entrance of the Dukhang, and dated to circa 1200 CE.

Traditionally, the creation of the Alchi complex is attributed to the famous scholar-translator Rinchen Zangpo (958–1055) in the 10th century, along with the Lamayuru Monastery, the Wanla, the Mang-gyu and the Sumda. During the tenth century, the Tibetan lama-king Yeshe-Ö of Guge, in order to spread Buddhism in the Trans Himalayan region, took the initiative by allocating 21 scholars to the region. However, due to harsh climatic and topographic conditions, only two survived, one of them the esteemed scholar and translator Rinchen Zangpo who established Buddhist activity in the Ladakh region and other areas of India including Tabo Gompa in Spiti, Himachal Pradesh, and Sikkim. During his sojourn there, he also went to the neighbouring countries of Nepal, Bhutan and Tibet. Zangpo became known by the epithet "Lochen," "Lotsawa Chenpo" or the "Great Translator"; he is credited with building 108 monasteries in the trans-Himalayan region in his quest to disseminate Buddhism. He institutionalized Buddhism in the region; these monasteries are considered the mainstay of Vajrayana Tibetan Buddhism. Zangpo engaged Kashmiri artists to create wall paintings and sculptures in the monasteries he is credited with building; only a few of these have survived. Out of these, the Alchi complex is remarkably well-preserved in terms of both art and architecture, and provides a rich history of both representative not only of Tibetan Buddhist tradition but of Persian styled courtly art.

Despite local tradition typically crediting Zangpo with Alchi's construction, this narrative is disputed based on cross-referenced inscriptions found throughout buildings of the complex. In the Sumtseg (Three-story temple), a partially damaged inscription known as the Garland of Butter Lamps (translated in full for the first time by Luczanits in 2023, based on Jaroslav Poncar's photographs), details the patrilineal genealogy of Sumtsek patron Tsültrim Ö, including references to an unnamed patrilineal uncle who the Sumtsek is dedicated to. A strikingly similar inscription in both prose and content, mentioning the same paternal uncle, is found in the Dukhang assembly hall, with the inscription itself attributed to Tibetan noble Kal-den Shes-rab. The genealogy found in these sets of inscriptions, corroborated by mentions of the king Wangdé, the late 11th century monarch of Guge, suggests that the Dukhang at Alchi was constructed by the uncle of Tsültrim Ö about a generation prior to the building of the Sumtseg, dating Alchi to (at very earliest) 1140 CE.