Tisha B'Av (Hebrew: תִּשְׁעָה בְּאָב Tīšʿā Bəʾāv; IPA: [tiʃʕa beˈʔav] (listen), lit. 'the ninth of Av') is an annual fast day in Judaism, on which a number of disasters in Jewish history occurred, primarily the destruction of both Solomon's Temple by the Neo-Babylonian Empire and the Second Temple by the Roman Empire in Jerusalem.

Tisha B'Av marks the end of the three weeks between dire straits and is regarded as the saddest day in the Jewish calendar, and it is thus believed to be a day which is destined for tragedy. Tisha B'Av falls in July or August in the Gregorian calendar.

The observance of the day includes five prohibitions, most notable of which is a 25-hour fast. The Book of Lamentations, which mourns the destruction of Jerusalem, is read in the synagogue, followed by the recitation of kinnot, liturgical dirges that lament the loss of the Temples and Jerusalem. As the day has become associated with remembrance of other major calamities which have befallen the Jewish people, some kinnot also recall events such as the murder of the Ten Martyrs by the Romans, expulsions from England, Spain and elsewhere, massacres of numerous medieval Jewish communities during the Crusades, and the Holocaust.