Bonsai (盆栽^?, lit. plantings in tray, from bon, a tray or low-sided pot and sai, a planting or plantings,  pronunciation (help·info)) is a Japanese art form using miniature trees grown in containers. Similar practices exist in other cultures, including the Chinese tradition of penjing from which the art originated, and the miniature living landscapes of Vietnamese hòn non bộ. The Japanese tradition dates back over a thousand years, and has its own aesthetics and terminology.

"Bonsai" is a Japanese pronunciation of the earlier Chinese term penzai. A "bon" is a tray-like pot typically used in bonsai culture. The word bonsai is often used in English as an umbrella term for all miniature trees in containers or pots, but this article focuses on bonsai as defined in the Japanese tradition.

The purposes of bonsai are primarily contemplation (for the viewer) and the pleasant exercise of effort and ingenuity (for the grower). By contrast with other plant cultivation practices, bonsai is not intended for production of food, for medicine, or for creating yard-size or park-size gardens or landscapes. Instead, bonsai practice focuses on long-term cultivation and shaping of one or more small trees growing in a container.

A bonsai is created beginning with a specimen of source material. This may be a cutting, seedling, or small tree of a species suitable for bonsai development. Bonsai can be created from nearly any perennial woody-stemmed tree or shrub species that produces true branches and can be cultivated to remain small through pot confinement with crown and root pruning. Some species are popular as bonsai material because they have characteristics, such as small leaves or needles, that make them appropriate for the compact visual scope of bonsai.