基础体温（英语：Basal body temperature，缩写：BBT），是人体在完全放松状态下，身体的最低温度，所以一般在睡眠后测量最为准确。测量应该在睡醒后立即进行，避免之前进行任何身体移动。通过基础体温的测量和记录，可以比较准确的了解女性的月经周期变化，预测排卵日和月经日。

对于一名正常的成年女性来说，基础体温会在排卵前下降0.1-0.2℃，排卵后立刻升高0.3-0.5℃，并持续到月经到来前1-2天，月经到来前，基础体温会下降到平均水平。而不正常的基础体温变化，通常表示身体的某种异常，比如已怀孕，早期流产，停止排卵，黄体功能不良，黄体素浓度不够等，需要及时就诊。

Basal body temperature (BBT) is the lowest body temperature attained during rest (usually during sleep). It is usually estimated by a temperature measurement immediately after awakening and before any physical activity has been undertaken. This will lead to a somewhat higher value than the true BBT (see Fig. 1). For more accurate results, it is determined by using internally worn temperature loggers. In women, ovulation causes an increase of one-half to one degree Fahrenheit (one-quarter to one-half degree Celsius) in BBT; monitoring of BBTs is one way of estimating the day of ovulation. The tendency of a woman to have lower temperatures before ovulation, and higher temperatures afterwards, is known as a biphasic pattern. Charting of this pattern may be used as a component of fertility awareness. The BBT of men is comparable to the BBT of women in their follicular phase.