An energy bar is a bar-shaped snack food usually marketed to athletes or other physically active people who wish to maintain their calorific needs in light of their strenuous physical activity. Some brands of energy bars are also sold as dietary supplements.

As the name indicates, they are mainly a source of food energy, primarily carbohydrates. Some bars contain a source of protein (often whey, casein, or soy), as well as a selection of added vitamins and minerals. The flavours added to the ingredients often make them resemble baked goods, such as cookies or muffins, to make them more appealing.

The packaging of a typical bar will provide an extended food label in order to assist the consumer in maintaining a dietary regimen. Major brands are available in most grocery stores in North America. It is also possible to make one's own energy bars, following a recipe or using a baking mix.

Energy bars, like all food, supply the body with physical energy, as measured in Calories or joules. Energy drinks, by contrast, are intended to improve mental energy by stimulating the central nervous system, usually with moderate to large doses of caffeineWarning: Energy Drinks Contain Caffeine by Allison Aubrey. Morning Edition, National Public Radio, 24 September 2008. or other stimulants, though many also supply physical energy in the form or sugar. Both energy bars and energy drinks often contain added vitamins.

Energy in food comes from three main sources: fat, protein, and carbohydrates. A typical energy bar will supply about , 3–9 g of fat, 7–15 g of protein, and 20–40 g of carbohydrates.

There are two types of carbohydrates: "simple" carbohydrates, which are digested quickly by the body, and "complex" carbohydrates, which take longer to digest. However, this term is misleading. For example, starches, often labeled as complex carbohydrates, are essentially stored glucose and are actually digested more quickly than sucrose and fructose, so-called "simple" carbohydrates. Any form of digestible carbohydrate will eventually cause a spike in blood sugar. Fiber is often added to energy bars to add bulk without calories and slow the absorption of glucose. It should be noted that insoluble fiber, like that found in whole grain wheat, does not do this. Many companies have started using it in energy bars to give consumers the feeling of satiety.

Many people over-indulge in carbohydrates, which can cause weight gain. When too many carbohydrates are eaten, the liver fills up with glycogen and still has glucose entering it. The liver will then store the excess glucose as fat, for later use. This is one reason why energy bars work better for athletes and people with active lifestyles.