The Fast of Esther (Ta'anith Ester, Hebrew: תַּעֲנִית אֶסְתֵּר) is a fast on Purim eve commemorating two communal fasts undertaken by the Persian Jewish community of Shushan in the Book of Esther, for the purpose of praying for salvation from annihilation by an evil decree, which had been the instigated by the royal vizier, an anti-jewish enemy of the Amalekite nation.

This fast, unlike other fasts, is a custom. It is not mentioned in the Talmud, however is mentioned in the Midrash and other later sources from the days of the Gaonim. Therefore it is considered less severe than the other fasts.

Esther asked Mordechai to have the Jews fast and pray on her behalf for 3 days and 3 nights, before she approached her husband, king Ahasuerus to beg for the life of her people. Another is presumed to have occurred on the 13th of Adar, the day before the Jews fought a battle to defend against their enemies, who had been given an irrevocable permission to murder Jews for one day a year on the 14th of that month, which is now a Jewish holiday of Purim on account of their victory.

Before approaching the king unbidden at risk of her life Esther told Mordechai (Esther 4:16)

Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day; I also and my maidens will fast in like manner; and so will I go in unto the king, which is not according to the law; and if I perish, I perish.