A bhikkhunī (Pali: 𑀪𑀺𑀓𑁆𑀔𑀼𑀦𑀻, Sanskrit: भिक्षुणी, romanized: bhikṣuṇī) is a Buddhist nun, fully ordained female in Buddhist monasticism. Bhikkhunīs live by the Vinaya, a set of either 311 Theravada, 348 Dharmaguptaka, or 364 Mulasarvastivada school rules. Until recently, the lineages of female monastics only remained in Mahayana Buddhism and thus were prevalent in countries such as China, Korea, Taiwan, Japan, and Vietnam, while a few women have taken the full monastic vows in the Theravada and Vajrayana schools. The official lineage of Tibetan Buddhist bhikkhunīs recommenced on 23 June 2022 in Bhutan when 144 nuns, most of them Bhutanese, were fully ordained.

According to the Buddhist Canon, women are as capable of reaching enlightenment as men. The Canon describes that the order of bhikkhunīs was first created by the Buddha at the specific request of his aunt and foster-mother Mahapajapati Gotami, who became the first ordained bhikkhunī. A famous work of the early Buddhist schools is the Therigatha, a collection of poems by elder nuns about enlightenment that was preserved in the Pāli Canon. The canon also describes extra vows required for women to be ordained as bhikkhunīs.

In the Vajrayana of Tibetan Buddhism, Guru Padmasambhava stated that being a woman was actually better than being a man:

"The basis for realizing enlightenment is a human body. Male or female – there is no great difference. But if she develops the mind bent on enlightenment, to be a woman is better."

The historical authorship of the controversial Eight Garudhammas cannot be traced to the Buddha. Written by others at a later date, it mandated the bhikkhunī order to be subordinate to and reliant upon the bhikkhu (monk) order. There are 253 Vinaya precepts for bhikkus. In places where the bhikkhunī lineage was historically absent or has died out due to hardship, alternative forms of renunciation have developed.

In Tibetan Buddhism, women first officially take refuge vows as a layperson. Then, the renunciate vows of rabtu jungwa (rab-jung) are given before the getsulma (Tibetan novice) ordination vows are given. After these, full bhikkhunī ordination may be given.

Theravadan women may choose to take an informal and limited set of vows similar to the historical vows of the getsulma (Sanskrit sāmaṇerī), like the maechi of Thailand and thilashin of Myanmar.