Seventeenth of Tammuz, the seventeenth day on the Hebrew calendar month of Tammuz, is a half-day (dawn to dusk) fast in Judaism. The fast commemorates different calamities that occurred. Jewish tradition teaches that on this day, the final protective walls of ancient Jerusalem were pierced by Rome, leading to the destruction of the Second Temple. Also, the Temple sacrifices were stopped and the tablets of the Ten Commandments were broken by Moses.

It is the second in the Four Fasts commemorating the destruction of the the Temple and the Jewish Exile. It is preceded by the fast of Tenth of Tevet seven months earlier, begins the three weeks before full-day fast of the Ninth of Av. The last of the four fasts is the Fast of Gedalia immediately following [[Rosh Hashanah.

The three weeks between the Seventeenth of Tamuz and the Ninth of Av are in themselves known as the Three Weeks, also known as bein hametzarim ("between the straits"), of a mounting sense of mourning for Jerusalem's and the Temples' destructions.