A fad diet is a diet that is popular, generally only for a short time, similar to fads in fashion, without being a standard scientific dietary recommendation. They often make unreasonable claims for fast weight loss or health improvements, and as such are often considered a type of pseudoscientific diet. Fad diets are usually not supported by clinical research and their health recommendations are not peer-reviewed, thus they often make unsubstantiated statements about health and disease.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 desired changes requiring little effort, thus attracting the interest of consumers uneducated about whole-diet, whole-lifestyle changes necessary for sustainable health benefits. 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 more dangerous claims achieved through highly restrictive and nutritionally unbalanced food choices leading to malnutrition or even eating non-food items such as 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 unsustainable, medically unsuitable to the individual, 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.

There is no single definition of what a fad diet is, encompassing a variety of diets with different approaches and evidence base, and thus different outcomes, advantages, and disadvantages. Furthermore, labeling a diet as a fad is ever-changing, varying socially, culturally, timely, and subjectively. However, a common definition lies in the popularity of a diet promoting short-term changes instead of lifelong changes, and that popularity (or lack thereof) has no association with a diet's effectiveness, nutritional soundness, or safety. The Federal Trade Commission defines fad diets as those that are highly restrictive and promoting energy dense foods that are often poor in nutrients.

Although fad diets are ever-changing, most can be categorized in these general groups:

Fad diets are generally restrictive, and are characterized by promises of fast weight loss or great physical health (notably by "detoxification"), and which are not grounded in sound science.

Some fad diets, such as diets purporting to be alternative cancer treatments, promise health benefits other than weight loss.

Commercial weight management organizations (CWMOs), such as Weight Watchers, were inappropriately associated with fad diets in the past.