Fast of the Firstborn (Hebrew: תענית בכורות, Ta'anit B'khorot or תענית בכורים, Ta'anit B'khorim) is a unique fast day in Judaism which usually falls on the day before Passover (i.e., the fourteenth day of Nisan, a month in the Jewish calendar; Passover begins on the fifteenth of Nisan). In modern times, the fast is usually broken at a siyum celebration (typically made at the conclusion of the morning services), which, according to prevailing custom, creates an atmosphere of rejoicing that overrides the requirement to continue the fast (see Breaking the fast below). Unlike all other Jewish fast days, only firstborn children are required to fast on the Fast of the Firstborn.

This fast commemorates the salvation of the Israelite firstborns during the Plague of the Firstborn (according to the Book of Exodus, the tenth of the ten plagues wrought upon Ancient Egypt prior to the Exodus of the Children of Israel), when, according to Exodus (12:29): "...God struck every firstborn in the Land of Mitzrayim (Ancient Egypt)...."