Fat fetishism is a sexual fetish in which sexual pleasure is derived from oneself or one's parter being overweight or obese. The weight ranges from being only mildly overweight or so obese as to be immobile and unable to walk. However, some argue that this is not a fetish and is simply a preference - as is liking, for example, women with large breasts or blonde hair.

The homosexual and heterosexual communities tend to parallel each other. In a basic fat fetish relationship, one or both partners are overweight. Often, the thin partner takes a dominant role due to the other's reduced mobility.

The relationship becomes more complex if feederism is involved.

Fat fetishism may at times overlap with breast fetish (especially large breasts), breast expansion fetish, inflation fetish and pregnancy fetish.

The fondling of the adipose tissue of an overweight or obese person for one with a fat fetish is usually sexually arousing and is often similar to that of mammary intercourse. They may be aroused by the presence of a pannus or other hanging flaps of skin such as those on the thighs, arms, back and other places. For those aroused by an obese male, the presence of fat under the chest simulating breasts (also known as "man boobs") is arousing.

The issue of erotic weight gain has caused a collision, at least in a heterosexually-dominated realm, between the goals of fat acceptance (represented by NAAFA and others) and feederism, which some contend are not mutually exclusive interests/movements. One argument mobilized against the feeder movement is that sexuality is privileged over social movements toward fat acceptance, and direct criticisms usually entail the possibility that feederism contributes to a counter-productive objectification and dehumanization of the "feedee". Feeders/feedees and sympathetic FAs argue that most members of their community partake of this fetish only in the realm of fantasy and are more than mindful of rational boundaries. Also present is a belief that the sexualization of fat is not as pernicious as its detractors claim, and that if the "Fat Admirer" or "Chubby-Chaser (male)" archetypes become more visible or mainstream, fat will thus lose a major aspect of its general social and cultural stigma.