Pescetarianism (/ˌpɛskəˈtɛəri.ənɪzəm/ PESK-ə-TAIR-ee-ə-niz-əm; sometimes spelled pescatarianism) is a dietary practice in which seafood is the only source of meat in an otherwise vegetarian diet. The inclusion of other animal products, such as eggs and dairy, is optional. According to research conducted from 2017 to 2018, approximately 3% of adults worldwide are pescetarian.

"Pescetarian" is a neologism formed as a portmanteau of the Italian word "pesce" ("fish") and the English word "vegetarian". The term was coined in the United Kingdom in the late 1980s. "Pesco-vegetarian" is a synonymous term that is seldom used outside of academic research, but it has sometimes appeared in other American publications and literature since at least 1980.

The first vegetarians in written western history may have been the Pythagoreans, a title derived from the Greek philosopher Pythagoras. Though Pythagoras loaned his name to the meatless diet, some biographers suspect he may have eaten fish as well at some points, which would have made him not a vegetarian but a pescatarian by today's standards. Many of Pythagoras's philosophies inspired Plato, who advocated for the moral and nutritional superiority of vegetarian-oriented diets. In Plato's ideal republic, a healthy diet would consist of cereals, seeds, beans, fruit, milk, honey and fish.

In 675, the consumption of livestock and wild animals was banned in Japan by Emperor Tenmu, due to the influence of Buddhism and the lack of arable land. However, Tenmu did not ban the consumption of deer or wild boar. Subsequently, in the year 737 of the Nara period, the Emperor Shōmu approved the eating of fish and shellfish. During the 1200 years from the Nara period to the Meiji Restoration in the latter half of the 19th century, Japanese people ate vegetarian-style meals, and on special occasions, seafood was served. Exceptions were wild fowl served amongst the Heian nobility, and when Europeans arrived in Japan in the 15th century, the Japanese diet included boar meat.

Several orders of monks in medieval Europe restricted or banned the consumption of meat for ascetic reasons, but none of them abstained from the consumption of fish; these monks were not vegetarians, but some were pescetarians.

Marcion of Sinope and his followers ate fish but no fowl or red meat. Fish was seen by the Marcionites as a holier kind of food. They consumed bread, fish, honey, milk, and vegetables.

The "Hearers" of the ecclesiastical hierarchy of Manichaeism lived on a diet of fish, grain, and vegetables. Consumption of land animals was forbidden, based on the Manichaean belief that "fish, being born in and of the waters, and without any sexual connexion on the part of other fishes, are free from the taint which pollutes all animals".

The Rule of Saint Benedict insisted upon total abstinence of meat from four-footed animals, except in cases of the sick. Benedictine monks thus followed a diet based on vegetables, eggs, milk, butter, cheese, and fish. Paul the Deacon specified that cheese, eggs, and fish were part of a monk's ordinary diet. Benedictine monk Walafrid Strabo commented, "Some salt, bread, leeks, fish and wine; that is our menu."