In Buddhism, the Eight Precepts (Sanskrit: aṣṭāṇga-śīla or aṣṭā-sīla, Pali: aṭṭhaṅga-sīla or aṭṭha-sīla) is a list of moral precepts that are observed by Nuns, or Upāsakas and Upasikās (lay Buddhists) on Uposatha (observance days) and special occasions. They are considered to support meditation practice, and are often observed when staying in monasteries and temples.

They include ethical precepts such as refraining from killing any living being, but also more specific ones, such as abstaining from entertainments. The tradition of keeping the Eight Precepts on weekly observance days is still widely practiced in all Theravadin Buddhist countries and communities worldwide. Based on pre-Buddhist sāmaṇa practices, the eight precepts are often upheld on the Buddhist observance days (Sanskrit: upavasatha, poṣadha, pauṣadha, Pali: uposatha, posaha), and in such context called the uposatha vows or one-day precepts. In some periods and places the precepts were widely observed, such as in 7th–10th-century China by government officials. In modern times, there have been revival movements and important political figures that have observed them continuously.

The first five of the eight precepts are similar to the five precepts, that is, to refrain from killing living beings, stealing, damaging speech, and to abstain from intoxicating drink or drugs, but the third precept is abstinence of all sexual activity instead of refraining from sexual offenses. The final three precepts are to abstain from eating at the wrong time (after midday); to abstain from entertainment such as dancing, singing, music, watching shows, as well as to abstain from wearing garlands, perfumes, cosmetics, and personal adornments; and to abstain from luxurious seats and beds.

To summarise, following anthropologist Barend Jan Terwiel's translation from the Pāli language used in Thai ceremonies:

In Thailand, when the eight precepts are taken, it is believed that if one of them is broken, they are all broken. In the Pāli tradition, the precepts are described in the Dhammika Sutta, part of the Sutta-Nipāta. In many medieval Chinese texts, the order of the last three items is different, with numbers 6 and 8 switched.

In the context of uposatha practice, observing the eight precepts is described by the Buddha in the early texts as "cleansing of the sullied mind through expedient means" (Pali: upakkamena). The Pāli texts describe that one undertakes the eight precepts on the observance days following the example of the enlightened disciples of the Buddha. In the early texts, the Buddha is described as drawing a distinction between the Buddhist and Jain ways of upholding the uposatha. The Jain way is criticized as being more focused on outward appearance than substance, and the Buddhist practice is dubbed as genuine moral discipline. The eight precepts are meant to give lay people an impression of what it means to live as a monastic, and the precepts "may function as the thin end of a wedge for attracting some to monastic life". People who are observing the eight precepts are sometimes also addressed differently. The objective of the eight precepts is different from the five in that they are less moral in nature, but more focused on developing meditative concentration, and preventing distractions. Indeed, in Sri Lanka, lay devotees observing the eight precepts are expected to spend much time and effort on meditation, focusing especially on meditation on the parts of the body. This is intended to develop detachment.

On regular observance days, Buddhist lay devotees often observe the eight precepts. In that context, the eight precepts are also called the uposatha vows (Sanskrit and Pali: upavāsa; Sanskrit: poṣadhaśīla, Pali: uposatha-sīla). When laypeople stay in a Buddhist monastery or go on a meditation retreat, they also observe the eight precepts often; they are also upheld during yearly festivals such as Vesak. Presently, the uposatha vows are mostly associated with Theravāda Buddhism in South and Southeast Asia, but it was a widespread practice in China as well, and is still practiced. In practice, in Theravāda traditions, the precepts are mostly observed by faithful devotees above 40 years of age. Since the eight precepts are often observed for one day, they are also known as the one-day precepts. Sometimes a formula is recited confirming the observance for one day (and one night):

"I undertake to observe in harmony during this day and this night these eight precepts that have been designed by the wisdom of the Buddha."