Marasmus is a form of severe malnutrition characterized by energy deficiency. It can occur in anyone with severe malnutrition but usually occurs in children. Body weight is reduced to less than 62% of the normal (expected) body weight for the age. Marasmus occurrence increases prior to age 1, whereas kwashiorkor occurrence increases after 18 months. It can be distinguished from kwashiorkor in that kwashiorkor is protein deficiency with adequate energy intake whereas marasmus is inadequate energy intake in all forms, including protein. This clear-cut separation of marasmus and kwashiorkor is however not always clinically evident as kwashiorkor is often seen in a context of insufficient caloric intake, and mixed clinical pictures, called marasmic kwashiorkor, are possible. Protein wasting in kwashiorkor generally leads to edema and ascites, while muscular wasting and loss of subcutaneous fat are the main clinical signs of marasmus, which makes the ribs and joints protrude.

The prognosis is better than it is for kwashiorkor. Marasmus is the form of nutrition most highly assosciated with HIV, developing in the last stages of pediatric AIDS, and the prognosis for children with co-morbid marasmus and HIV is very poor.

The word "marasmus" comes from the Greek μαρασμός marasmos ("withering").