Kol Nidre (also known as Kol Nidrey or Kol Nidrei; Aramaic: כָּל נִדְרֵי kāl niḏrē) is a Hebrew and Aramaic declaration which is recited in the synagogue before the beginning of the evening service on every Yom Kippur ("Day of Atonement"). Strictly speaking, it is not a prayer, even though it is commonly spoken of as if it is a prayer. This declaration and its ceremonial accompaniment have been charged with emotional undertones since the medieval period, creating a dramatic introduction to Yom Kippur on what is often dubbed "Kol Nidrei night", with the entire Yom Kippur evening service popularly called Kol Nidrei.

Kol Nidrei is written in a mixture of Aramaic and Hebrew. Its name is taken from its opening words, words which mean "all vows". The formula proactively annuls any personal or religious oaths or prohibitions which are made between oneself and God for the next year, so as to preemptively avoid the sin of breaking vows which are made to God but are not or cannot be upheld.

Kol Nidrei has had an eventful history, both in itself and in its influence on the legal status of the Jews. Introduced into the liturgy, it was attacked by some rabbis (halakhists), and in the 19th century, it was expunged from the prayer book by many communities in western Europe.