The rainy season, or monsoon season, is the time of year when most of the average annual precipitation of a region occurs. It usually lasts for one or more months. The term "green season" is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed through portions of the tropics and subtropics.

Under Köppen's climatic classification, for tropical climates, a wet season month is defined as a month in which the average rainfall is 60 millimeters (2.4 inches) or more. In contrast to areas with savannah climates and monsoon regimes, Mediterranean climates have wet winters and dry summers. The dry and rainy months are characteristic of seasonal tropical forests: in contrast to tropical rainforests, which do not have dry or humid seasons, as rainfall is equally distributed throughout the year. Some zones with pronounced rainy seasons will see a mid-season precipitation interruption, when the intertropical convergence zone or the monsoon valley moves to higher latitudes in the middle of the warm season.

When the wet season occurs during a warm season, or summer, precipitation falls mainly during the afternoon and early afternoon. In the wet season, air quality improves, freshwater quality improves, and vegetation grows substantially, leading to crop yields at the end of the season. Rivers overflow their banks, and some animals retreat to higher ground. Soil nutrients decrease and erosion increases. The incidence of malaria increases in areas where the rainy season coincides with high temperatures, particularly in tropical areas. Some animals have adaptive and survival strategies for the wet season. Often, the previous dry season leads to food shortages in the wet season, as the crops have not yet matured.