Breakfast cereal is a traditional breakfast food made from processed cereal grains. It is traditionally eaten as part of breakfast, or a snack food, primarily in Western societies.

Although warm cereals like porridge and grits have the longest history, ready-to-eat cold cereals appeared around the late 19th century, and are most often mixed with milk (traditionally cow's milk), but can also be paired with yogurt instead or eaten plain. Fruit or nuts are sometimes added. Many cereals are produced via extrusion.

Some companies promote their products for the health benefits that come from eating oat-based and high-fiber cereals. In the United States, cereals are often fortified with vitamins but can still lack many of the vitamins needed for a healthy breakfast. A significant proportion of cereals have a high sugar content ("sugar cereals" or "sugary cereals" in common parlance). Some cereals are marketed primarily towards children, feature a cartoon mascot, and may contain a toy or prize.

Between 1970 and 1998, the number of different types of breakfast cereals in the U.S. more than doubled, from about 160 to around 340; as of 2012, there were roughly 4,945 different types (estimate based on the mass customization of online shopping). In this highly competitive market, cereal companies have developed an ever-increasing number of varieties and flavors (some are flavored like dessert or candy). Although many plain wheat-, oat- and corn-based cereals exist, many other varieties are highly sweetened, while some brands include freeze-dried fruit as a sweet element. The breakfast cereal industry has gross profit margins of 40–45%, 90% penetration in some markets, and has had steady and continued growth throughout its history.