Bagan (/bəˈɡæn/ bə-GAN; Burmese: ပုဂံ Băgam [bəɡàɰ̃]; formerly Pagan) is an ancient city and a UNESCO World Heritage Site in the Mandalay Region of Myanmar. From the 9th to 13th centuries, the city was the capital of the Pagan Kingdom, the first kingdom that unified the regions that would later constitute Myanmar. During the kingdom's height between the 11th and 13th centuries, more than 10,000 Buddhist temples, pagodas and monasteries were constructed in the Bagan plains alone, of which the remains of over 2200 temples and pagodas survive.

The Bagan Archaeological Zone is a main attraction for the country's nascent tourism industry.

Bagan is the present-day standard Burmese pronunciation of the Burmese word Pugan (ပုဂံ), derived from Old Burmese Pukam (ပုကမ်). Its classical Pali name is Arimaddanapura (အရိမဒ္ဒနာပူရ, lit. "the City that Tramples on Enemies"). Its other names in Pali are in reference to its extreme dry zone climate: Tattadesa (တတ္တဒေသ, "parched land"), and Tampadīpa (တမ္ပဒီပ, "bronzed country"). The Burmese chronicles also report other classical names of Thiri Pyissaya (သီရိပစ္စယာ; Pali: Siripaccaya) and Tampawaddy (တမ္ပဝတီ; Pali: Tampavatī).

According to the royal chronicles, Bagan was founded in the second century CE, and fortified in 849 by King Pyinbya, 34th successor of the founder of early Bagan. Western scholarship however holds that Bagan was founded in the mid-to-late 9th century by the Mranma (Burmans), who had recently entered the Irrawaddy valley from the Nanzhao Kingdom. It was among several competing Pyu city-states until the late 10th century when the Burman settlement grew in authority and grandeur.

From 1044 to 1287, Bagan was the capital as well as the political, economic and cultural nerve center of the Bagan Empire. Over the course of 250 years, Bagan's rulers and their wealthy subjects constructed over 10,000 religious monuments (approximately 1000 stupas, 10,000 small temples and 3000 monasteries) in an area of 104 km² (40 sq mi) in the Bagan plains. The prosperous city grew in size and grandeur, and became a cosmopolitan center for religious and secular studies, specializing in Pali scholarship in grammar and philosophical-psychological (abhidhamma) studies as well as works in a variety of languages on prosody, phonology, grammar, astrology, alchemy, medicine, and legal studies. The city attracted monks and students from as far as India, Sri Lanka and the Khmer Empire.

The culture of Bagan was dominated by religion. The religion of Bagan was fluid, syncretic and by later standards, unorthodox. It was largely a continuation of religious trends in the Pyu era where Theravada Buddhism co-existed with Mahayana Buddhism, Tantric Buddhism, various Hindu (Saivite, and Vaishana) schools as well as native animist (nat) traditions. While the royal patronage of Theravada Buddhism since the mid-11th century had enabled the Buddhist school to gradually gain primacy, other traditions continued to thrive throughout the Pagan period to degrees later unseen.

Bagan's basic physical layout had already taken shape by the late 11th century, which was the first major period of monument building. A main strip extending for about 9 km along the east bank of the Irrawaddy emerged during this period, with the walled core (known as "Old Bagan") in the middle. 11th-century construction took place throughout this whole area and appears to have been relatively decentralized. The spread of monuments north and south of Old Bagan, according to Hudson, Nyein Lwin, and Win Maung, may reflect construction at the village level, which may have been encouraged by the main elite at Old Bagan.

The peak of monument building took place between about 1150 and 1200. Most of Bagan's largest buildings were built during this period. The overall amount of building material used also peaked during this phase. Construction clustered around Old Bagan, but also took place up and down the main strip, and there was also some expansion to the east, away from the Irrawaddy.