Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues.

Temperature control (thermoregulation) is part of a homeostatic mechanism designed to keep the organism at optimum operating temperature, as it affects the rate of chemical reactions. In humans this optimum temperature is 36.8 °C (98.2 °F) (see Normal human body temperature), though it varies regularly as controlled by one's circadian rhythms with the lowest temperature occurring about two hours before one normally wakes up. An organism at optimum temperature is considered afebrile or apyrexic.

Temperature examination in the rectum is the traditional gold standard measurement used to estimate core temperature (oral temperature is affected by hot or cold drinks and mouth-breathing). The recent introduction of ear temperature measurement may also accurately reflect core body temperature particularly since the eardrum shares blood supply with the temperature control center in the brain, the hypothalamus.