A humid subtropical climate is a climate type located within the temperate climate type, characterized by long, hot and humid summers, and cool to mild winters. Subtropical climates, the warmest of the temperate zone climates, normally lie on the southeast side of all continents (except Antarctica), generally between latitudes 20° and 35° and are located poleward from adjacent tropical climates, and equatorward from either humid continental (in North America and Asia) or oceanic climates (in other continents).

Under the Köppen climate classification, Cfa and Cwa climates are described as warm temperate climates when mean temperature in the coldest month is above 0 °C (32 °F) and below 18 °C (64 °F). However, some climatologists have opted to describe the most southernmost portion of the temperate zone as "humid subtropical climate". In this southernmost zone, normally the mean temperature of the coldest month is 45 F (7.6 C) or higher and has mean temperature in the hottest months above 75 F (23.8 C) , In the Köppen climate classification, this southernmost portion of the temperate zone was not defined separately.

The humid subtropical climate classification was a later revision under the Trewartha climate classification. In this classification, climates are termed humid subtropical when they have at least 8 months with a mean temperature above 10 °C (50 °F). This distinction normally separates the general temperate zone from the southernmost portions of the temperate zone which have many tropical climate characteristics for a portion of the year.

While many subtropical climates tend to be located on the eastern lowlands and coastal areas of landmasses, in some cases, they extend well inland and to a higher latitude, most notably in China and the interior southern United States, where they exhibit more pronounced seasonal variations and sharper contrasts between summer and winter. As such, the climate can be said to exhibit somewhat different features depending from the hotter, more mild winter coastal and coastal plain locations.

In a humid subtropical climate, summers are typically long, hot and humid. A deep current of tropical air dominates the humid subtropics at the time of high sun, and daily intense (but brief) convective thundershowers are common. Monthly mean temperatures in winter are normally mild, however they may be cool at the northern edges of the humid subtropical zone.

Rainfall often shows a summer peak in most humid subtropical climates, especially where there is a well developed monsoon (seasonal reversal of winds) such as in South Asia. Most summer rainfall occurs during thunderstorms that build up due to the intense surface heating and strong subtropical sun angle. Weak tropical lows that move in from adjacent warm tropical oceans, as well as occasional tropical cyclones often contribute to summer seasonal rainfall peaks such as occurs in the Florida in the United States and eastern Australia. Normally, winter is a drier season in most humid subtropical climates, however, some higher latitude subtropical climates in the interior of the southern United States, also have modest precipitation in winter as well.

Under the Holdridge life zones classification, the subtropical climates have a biotemperature between the frost or critical temperature line, 16 to 18 °C (61 to 64 °F) (depending on locations in the world) and 24 °C (75 °F), and these climates are humid (or even perhumid or superhumid) when the potential evapotranspiration (PET) ratio (= PET / Precipitation) is less than 1. In the Holdridge classification, the humid subtropical climates of the Koppen system coincide more or less with the subtropical and warm temperate life zones.

Cfa: C = Mild temperate f = Fully humid a = Hot summer