A fad diet is a diet that becomes popular for a short time, similar to fads in fashion, without being a standard dietary recommendation, and often making pseudoscientific or unreasonable claims for fast weight loss or health improvements. Fad diets are not supported by clinical research and their health recommendations are not peer-reviewed, thus they often make unsubstantiated statements about health and disease.

Generally, fad diets promise an assortment of short-term changes requiring little to no effort; attracting the interests of consumers uneducated about whole-diet, whole-lifestyle changes necessary for sustainable health benefices. Fad diets are often promoted with exaggerated claims, such as rapid weight loss of more than 1 kg/week, improving health by "detoxification", or even dangerous claims, such as highly restrictive and nutritionally unbalanced food choices leading to malnutrition or eating non-food items like cotton wool. Highly restrictive fad diets should be avoided. At best, fad diets may offer novel and engaging ways to reduce caloric intake, but at worst they may be medically unsuitable to the individual, unsustainable, or even dangerous. Dietitian advice should be preferred before attempting any diet.

Celebrity endorsements are frequently used to promote fad diets, which may generate significant revenue for the creators of the diets through the sale of associated products. Regardless of their evidence base, or lack thereof, fad diets are extremely popular, with over 1500 books published each year, and many consumers willing to pay into an industry worth $35 billion per year in the United States. About 14–15% Americans declare having used a fad diet for short-term weight loss.