Loy Krathong (Thai: ลอยกระทง, RTGS: Loi Krathong, pronounced [lɔ̄ːj krā.tʰōŋ]) is a Thai festival celebrated annually throughout Thailand and in nearby countries with significant South Western Tai cultures (Laos, Shan, Mon, Tanintharyi, Kelantan, Kedah, Perlis and Xishuangbanna). The name could be translated as "to float ritual vessel or lamp," and comes from the tradition of making krathong or buoyant, decorated baskets, which are then floated on a river. Many Thais use the krathong to thank the Goddess of Water and River, Goddess Khongkha (Thai: พระแม่คงคา) This festival traces its origin back to India.

Loy Krathong takes place on the evening of the full moon of the 12th month in the traditional Thai lunar calendar, thus the exact date of the festival changes every year. In the Western calendar this usually falls in the month of November. In Chiang Mai, the festival lasts three days, and in 2024, the dates are 15-17 November.

In Thailand, the festival is known as Loi Krathong. Outside Thailand, this festival is celebrated under different names, including Myanmar as the "Tazaungdaing festival", Sri Lanka as "Il Full Moon Poya", China as "Lantern Festival" and Cambodia as "Bon Om Touk".

A krathong is traditionally a small floating container fashioned of leaves which is made to hold a small portion of goods like a traditional Thai dish (such as hor mok) or dessert. The traditional krathong used for floating at the festival are made from a slice of a banana tree trunk or a spider lily plant. Modern krathongs are more often made of bread or Styrofoam. A bread krathong will disintegrate after a few days and can be eaten by fish. Banana stalk krathongs are also biodegradable, but Styrofoam krathongs are increasingly banned, as they pollute rivers and oceans. A krathong is decorated with elaborately folded banana leaves, three incense sticks, and a candle. A small coin is sometimes included as an offering to the river spirits. On the night of the full moon, Thais launch their krathong on a river, canal, or a pond, making a wish as they do so. The floats are thought to have been introduced to Chiang Mai in 1947 and have since been incorporated into Thai culture. Government offices, corporations, and other organizations launch large decorated krathongs. There are competitions for the best of these large krathongs. Beauty contests often accompany the festivities and fireworks also have become common.

The practice of Loy Krathong in Thailand which roots Hindu in origin, derived from Dewali, the festival of lights celebrated in India as Hindus were known to celebrate the Diwali festival to worship the divine trinity and those Hindus who were converted to Buddhism had created a rite similar to the Diwali. Hinduism has existed in Suvarnabhumi in a syncretistic relationship with Theravada Buddhism at least since 5th century B.C.

In Khmer Empire. Walls of Bayon, a temple built by King Jayavarman VII in the 12th century, depicts scenes of Loy Krathong. A bas relief on the upper level depicts a queen residing on the boat to float the krathong in the river whereas six other royal concubines are depicted below, some of which are holding the krathong and dedicating at the riverbank in a similar tradition practiced in present-day Cambodia, Thailand, and other Southeast Asian countries. Similarities can be observed with Kartik Purnima celebrated in the eastern state of Odisha in India. This festival is called Boita Bandana\ Dangā Bhasā, which is observed on the Kartik Purnima or full moon day of Kartik month (which corresponds to October–November) in Odia calendar. Loy Krathong festivities are usually celebrated during the period which corresponds to Kartik Purnima. Odisha being part of the ancient Kalinga which had strong maritime trade relations with Southeast Asia, the similarities in all these festivals may not be coincidental.

According to the 1999 Royal Institute Dictionary, loi (ลอย) means 'to float', while krathong (กระทง) has various meanings, one of which is 'a small vessel made of leaves which can be floated on water during the Loy Krathong festival.' Moreover, according to the Royal Society of Thailand, the word krathong is derived from Old Chinese word 鐙 or 燈 (/*k-tˤəŋ/) which means ritual vessel or lamp. However, other sources believe krathong to be a derivate of the Malay word kantong (Kantong), which has a similar pronunciation and the same meaning.

Loy Krathong is once said to have begun in the Sukhothai Kingdom by a court lady named Nopphamat. However, it is now known that the Nopphamat tale comes from a poem written in the early-Bangkok period. According to King Rama IV, writing in 1863, it was a Hindu festival that was adapted by Thai Buddhists in Thailand to honour the Buddha, Prince Siddhartha Gautama. The candle venerates the Buddha with light, while the krathong's floating away symbolises letting go of all one's hatred, anger, and defilements. People sometimes cut their fingernails or hair and place the clippings on the krathong as a symbol of letting go of past transgressions and negative thoughts. Many Thais use the krathong to thank the Goddess of Water, the Hindu Goddess Ganga, Phra Mae Khongkha (Thai: พระแม่คงคา).