Low-carbohydrate diets or low-carb diets are dietary programs that restrict carbohydrate consumption usually for weight control or for the treatment of obesity. Foods high in digestible carbohydrates (e.g. bread, pasta) are limited or replaced with foods containing a higher percentage of proteins and fats (e.g., meat, soy products) and often other foods low in carbohydrates (e.g., green leafy vegetables).

The American Academy of Family Physicians provides the following definition of low-carbohydrate diets.

Low-carbohydrate diets restrict caloric intake by reducing the consumption of carbohydrates to 20 to 60 g per day (typically less than 20 percent of the daily caloric intake). The consumption of protein and fat is increased to compensate for part of the calories that formerly came from carbohydrates.

This definition is typical of most sources although no universally recognized definition has been established. Such diets are generally ketogenic (i.e. they restrict carbohydrate intake sufficiently to cause ketosis) for example, the induction phase of the Atkins diet. Some sources, though, consider less restrictive variants to be low-carbohydrate as well.

Apart from obesity, low-carbohydrate diets are often discussed as treatments for some other conditions, most notably diabetes and epilepsy, although, other than for intractable epilepsy in children, these treatments still remain controversial and lack widespread support.