A kigo (季語; 'season word') is a word or phrase associated with a particular season, used in traditional forms of Japanese poetry. Kigo are used in the collaborative linked-verse forms renga and renku, as well as in haiku, to indicate the season referred to in the stanza. They are valuable in providing economy of expression.

Representation of, and reference to, the seasons has long been important in Japanese culture and poetry. The earliest anthology of Japanese poetry, the mid-8th century Man'yōshū, contained several sections devoted to the seasons. By the time of the first imperial Japanese anthology, the Kokinshū a century and a half later (AD 905), the seasonal sections had become a much larger part of the anthology. Both of these anthologies had sections for other categories such as love poems and miscellaneous (zō) poems.

The writing of the linked-verse form renga dates to the middle of the Heian period (roughly AD 1000) and developed through the medieval era. Over time, set rules developed for the writing of renga, and its formal structure specified that about half of the stanzas should include a reference to a specific season, depending upon their place in the poem. According to these rules, the hokku (the opening stanza of the renga) must include a reference to the season in which the renga was written. Poets as early as Iio Sogi (1421-1502) introduced the concept of seasonal references with anthologies of seasonal topics.

A lighter form of renga called haikai no renga ("playful" linked verse) was introduced in the 16th century, and became a salon type recreation by the Tokugawa era. Poets soon began to compose hokku independent of the longer, collaborative renga and it began to become an independent style. In the early twentieth century poets began experimenting with breaking the traditional elements of haiku, such as omitting the kigo entirely. This eventually led to the New Haiku and free verse haiku movements, which advocated more modern styles of haiku. Today most Japanese haiku include a kigo, though many haiku written in languages other than Japanese omit it (see for example Haiku in English).

Season words are evocative of images that are associated with the same time of year. For readers in New England, a poem about frost on a pumpkin evokes other sensations and traditions, like frosty air and apple cider. For Higginson writing in The Haiku Seasons, season words are a type of logopoeia--a word used not just for its meaning, but for its associations with other ideas.

The association of kigo with a particular season may be obvious, though sometimes it is more subtle. In Japan, Pumpkins (kabocha) are a winter squash associated with the autumn harvest. It may be less obvious why the moon (tsuki) is an autumn kigo, since it is visible year round. In autumn the days become shorter and the nights longer, yet they are still warm enough to stay outside, so one is more likely to notice the moon. Often, the night sky will be free of clouds in autumn, with the moon visible. The full moon can help farmers work after the sun goes down to harvest their crops (a harvest moon).

Japan is long from north to south, so the seasonal features vary from place to place. The sense of season in kigo is based on the region between Kyoto and Tokyo, because Japanese classical literature developed mainly in this area. In the Japanese calendar, seasons traditionally followed the lunisolar calendar with the solstices and equinoxes at the middle of a season. The traditional Japanese seasons are:

Within season categories, kigo can denote early, middle, or late parts of a season, which are defined approximately as the first, second, or third month of the season. In linked haiku forms like renku, subsequent linked haiku must move forward in season temporally. There are other rules governing season words in renku, including the frequency of certain season words, and how many stanzas remain in a season, once mentioned.