Niddah (or nidah; Hebrew: נִדָּה), in traditional Judaism, describes a woman who has experienced a uterine discharge of blood (most commonly during menstruation), or a woman who has menstruated and not yet completed the associated requirement of immersion in a mikveh (ritual bath).

In the Book of Leviticus, the Torah prohibits sexual intercourse with a niddah. The prohibition has been maintained in traditional Jewish law and by the Samaritans. It has largely been rejected by adherents of Reform Judaism and other liberal branches.

In rabbinic Judaism, additional stringencies and prohibitions have accumulated over time, increasing the scope of various aspects of niddah, including: duration (12-day minimum for Ashkenazim, and 11 days for Sephardim); expanding to prohibition against sex to include: sleeping in adjoining beds, any physical contact, and even passing objects to spouse; and requiring a detailed ritual purification process.

Since the late 19th century, with the influence of German Modern Orthodoxy, the laws concerning niddah are also referred to as Taharat haMishpacha (טָהֳרַת הַמִּשְׁפָּחָה‎, Hebrew for family purity), an apologetic euphemism coined to de-emphasize the "impurity" of the woman (a concept criticized by the Reform movement) and to exhort the masses by warning that niddah can have consequences on the purity of offspring.