Kitniyot (Hebrew: קִטְנִיּוֹת, qitniyyot) is a Hebrew word meaning legumes. During the Passover holiday, however, the word kitniyot (or kitniyos in Ashkenazi dialects) takes on a broader meaning to include grains and seeds such as rice, corn, sunflower seeds, and sesame seeds, in addition to legumes such as beans, peas, and lentils.

The Torah prohibits Jews from eating chametz during Passover. Chametz is defined as leaven made from the "five species of grain" (wheat, barley, and three similar grains). Food made from any other species is not considered chametz. However, among Ashkenazi and some Sephardic customs, the custom (minhag) during Passover is to refrain from not only products of the five grains but also other grains and legumes, known as kitniyot, even though they are not chametz.

Traditions of what is considered kitniyot vary from community to community but generally include maize (American corn), as well as rice, peas, lentils, and beans. Many also include other legumes, such as peanuts and soy, in this prohibition. The Chayei Adam considers potatoes not to be kitniyot because they were unknown in the time when the prohibition was created, an opinion followed today by nearly all Ashkenazi authorities.

Some Sephardic and Yemenite Jews have not traditionally observed a prohibition on eating kitniyot on Passover, although some groups do abstain from the use of dried pulses during Passover.

Since wheat flour only becomes chametz after it is ground and then mixed with water, one might assume that the kitniyot custom does not forbid kitniyot that were never ground or never came in contact with water. By this logic, it might be permitted to eat fresh kitniyot (like whole beans), or processed kitniyot which never came in contact with water (like certain squeezed oils or toasted solids). In fact, Rabbi Mordechai Eliyahu stated that the "first Ashkenazim in Jerusalem before the establishment of the state allowed fresh legumes and only prohibited dry legumes, but when the students of the Vilna Gaon and Baal Shem Tov came to Israel, they ‘brought with them’ from Europe the prohibition against fresh legumes". Conservative rabbis have ruled to permit fresh kitniyot.

In the 1930s, Maxwell House coffee hired the Joseph Jacobs advertising firm to market to a Jewish demographic. The agency hired a rabbi to research coffee, resulting in a determination that the coffee bean is more like a berry than a bean, thus making it kosher for Passover.

The Halakhic argument (the argument according to Jewish law and tradition) against eating kitniyot during Passover originated in early medieval France and Provence and later flourished in high medieval Ashkenazi (Rhineland) Germany. Most rabbinic sources prior to the 13th century, including the writings of Rav Huna (3rd century), Rava (4th century), Rav Ashi (5th century), and Maimonides (11th century), explicitly allowed eating kitniyot during Passover.

The original reasons behind the custom of not eating kitniyot during Passover are not clear. Suggestions include: