动情周期，又名动情周期，是雌性有胎盘哺乳类动物拥有的一种经常性生理变化，由身体的性激素所诱导产生。自前一次排卵期至下一次排卵期之间的时间长度称为一个周期。动物体内的各种激素浓度还有动物的生育能力都随动情周期变化。各种动物的动情周期长度不同。人类的动情周期现象称为月经，长度大约是28日。

动情周期在性成熟的青春期雌性开始出现，至休情期或者怀孕期间暂时停止，直到下一周期开始。这个循环会一直持续，直到死亡。有些动物在动情周期时，阴道会有带血的分泌物，而这些分泌物有时会被误以为是月经。

The estrous cycle (also oestrus cycle; derived from Latin oestrus and originally from Greek οἶστρος meaning sexual desire) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian therian females. Estrus cycles start after sexual maturity in females and are interrupted by anestrous phases or pregnancies. Typically, estrous cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation, also called a "period".