The wai (Thai: ไหว้, pronounced [wâːj]) is a greeting in Thailand that consists of a slight bow, with the palms pressed together in a prayer-like fashion. It has its origin in the Indian Añjali Mudrā, like the Indian namaste and Burmese mingalaba. The higher the hands are held in relation to the face and the lower the bow, the more respect or reverence the giver of the wai is showing. The wai is traditionally observed upon formally entering a house. After the visit is over, the visitor asks for permission to leave and repeats the salutation made upon entering. The wai is also common as a way to express gratitude or to apologise.

The wai gesture originated in Buddhism and has similar origins as namaste in Hinduism. It was basically a yogic posture of the palms and signifies the equal meeting of the two palms. It means that the other party is treated as an equal human being.

The word often spoken with the wai as a greeting or farewell is "sawatdi" (RTGS for สวัสดี, pronounced [sā.wàt.dīː], sometimes romanized as sawasdee). This verbal greeting is usually followed by "kha" when spoken by a female and by "khrap" when spoken by a male person (see note on Thai polite particles). The word sawatdi was coined in the mid-1930s by Phraya Upakit Silapasan of Chulalongkorn University. Derived from the Sanskrit svasti (स्वस्ति meaning 'well-being'), it had previously been used in Thai only as a formulaic opening to inscriptions. The strongly nationalist government of Plaek Phibunsongkhram in the early–1940s promoted its use in the government bureaucracy as well as the wider populace as part of a wider set of cultural edicts to modernise Thailand.

Waiing remains to this day an extremely important part of social behavior among Thais, who are very sensitive to their self-perceived standing in society. It is also frequently used as an accompaniment to an apology, sometimes even serving as a "get out of jail free card". Foreign tourists and other visitors unaccustomed to the intricacies of Thai language and culture should not wai someone younger than them except in return for their wai. However, one should always return a wai that is offered as a sign of respect. Corporate wais, such as those performed by convenience store cashiers, generally are reciprocated with a smile or a nod.

If one receives a wai while carrying goods, or for any reason that makes returning it difficult, one should still show their respect by making a physical effort to return it as well as possible under the circumstances.

Pranāma or Namaste, the part of ancient Indian culture has propagated to Southeast Asia, which was part of indosphere of greater India, through the spread of Hinduism and Buddhism from India. It has influenced the following nations.

In China, similar greetings—called suk sabaidee by Tai Lue people (Xishuangbanna) and ᥛᥬᥰ ᥕᥧᥱ ᥘᥤ ᥕᥧᥱ ᥔᥣ? by Tai Nua people (Dehong Dai).

In Cambodia, Laos, and Myanmar, similar greetings—called sampeah (សំពះ), sabaidee (ສະບາຍດີ), mingalaba (မင်္ဂလာပါ), maur sung kha (မႂ်ႇသုင်ၶႃႈ) (Shan people) respectively—are also in use.