Orthodox Judaism is the collective term for the traditionalist branches of contemporary Judaism. It is the most religiously stringent of the three main streams of American Judaism. Orthodox Jews believe in a strict interpretation of Jewish law, which they think is grounded in the Torah, both oral and written, and the revelation made to Moses by God on Mount Sinai. They view themselves as the continuation of the beliefs and practices of normative Judaism, as accepted by the Jewish nation at Mt. Sinai and codified in successive generations in an ongoing process that continues to this day.

Orthodox Judaism is not a centralized denomination, and relations between its different subgroups are sometimes strained. It is an umbrella term that covers multiple highly distinct subgroups, including Modern Orthodox, ultra-Orthodox (also known as haredi Orthodox), and Hasidic. Each of those is itself formed of independent communities.

Orthodox Jews follow the laws of the Torah and only eat Kosher foods. They may eat any animal with cloven hooves and chews its cud, so cattle, sheep, goats, deer, and bison are kosher. Pork and shellfish are forbidden, but fish with fins and scales are allowed. Fowl such as chicken, geese, ducks, and turkey are allowed. The mammals and birds must be slaughtered in accordance with Jewish law. Fruits and vegetables are kosher but need to be washed and free of bugs. Orthodox Jews are prohibited from eating meat and dairy together.

Orthodox Jews are more likely than other Jews to believe in God with absolute certainty and participate in various Jewish religious practices. They tend to identify as Republicans and take conservative positions on social issues such as homosexuality. On average, they also are more religiously committed and much younger than other U.S. Jews, and they have bigger families.