Which SI derived unit is used for entropy?

The SI derived unit used for entropy is the joule per kelvin (J/K).

Entropy has the dimensions of energy divided by temperature. In classical thermodynamics, Rudolf Clausius formalized entropy in the 19th century through the relation dS = δQ_rev/T, linking entropy change to reversible heat transfer and absolute temperature. Because heat is measured in joules and temperature in kelvins, entropy is measured in joules per kelvin.

Entropy is often confused with “disorder,” but that shorthand is incomplete. In statistical mechanics, it is related to the number and probability of microscopic arrangements available to a system. Entropy may also be reported per kilogram or per mole, giving J/(K·kg) or J/(K·mol); those are specific or molar quantities, not the SI unit of total entropy. The second law says that the entropy of an isolated system does not decrease during spontaneous evolution, increasing for irreversible processes.