The Trewartha climate classification (TCC), or the Köppen–Trewartha climate classification (KTC), is a climate classification system first published by American geographer Glenn Thomas Trewartha in 1966. It is a modified version of the Köppen–Geiger system, created to answer some of its deficiencies. The Trewartha system attempts to redefine the middle latitudes to be closer to vegetation zoning and genetic climate systems.

Trewartha's modifications to the 1884 Köppen climate system sought to reclass the middle latitudes into three groups, according to how many months have a mean temperature of 10 °C (50 °F) or higher:

The tropical climates and polar climates remained the same as in the original Köppen climate classification.

The "highland" climate is ambiguously defined. Newer users of KTC generally omit this option.

This is the tropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average 18 °C (64.4 °F) or above). The A climates are the realm of the winterless frost-free zone.

There was no specific monsoon climate identifier in the original scheme, but Am was added later, with the same parameters as Köppen's (except that at least three months, rather than one, must have less than 60 mm average precipitation).

BW and BS mean the same as in the Köppen scheme. However, a different formula is used to quantify the aridity threshold: 10(T − 10) + 3P, with T equaling the mean annual temperature in degrees Celsius, and P denoting the percentage of total precipitation received in the six high-sun months (April through September in the Northern Hemisphere, October through March in the Southern).

Unlike in Köppen's scheme, no thermal subsets exist within this group in Trewartha's, unless the Universal Thermal Scale (see below) is used.