A macrobiotic diet (or macrobiotics) is an unconventional restrictive diet based on ideas about types of food drawn from Zen Buddhism. The diet tries to balance the supposed yin and yang elements of food and cookware. Major principles of macrobiotic diets are to reduce animal products, eat locally grown foods that are in season, and consume meals in moderation.

There is no high-quality clinical evidence that a macrobiotic diet is helpful for people with cancer or other diseases, and it may be harmful. Neither the American Cancer Society nor Cancer Research UK recommends adopting the diet. Deaths have been reported from malnutrition on strict macrobiotic diets.

The macrobiotic diet is associated with Zen Buddhism and is based on the idea of balancing yin and yang. The diet proposes ten plans which are followed to reach a supposedly ideal yin:yang ratio of 5:1. The diet was popularized by George Ohsawa in the 1930s and subsequently elaborated on by his disciple Michio Kushi. Medical historian Barbara Clow writes that, in common with many other types of quackery, macrobiotics takes a view of illness and of therapy which conflicts with mainstream medicine.

Macrobiotics emphasizes locally grown whole grain cereals, pulses (legumes), vegetables, edible seaweed, fermented soy products, and fruit combined into meals according to the ancient Chinese principle of balance known as yin and yang. Some macrobiotic proponents stress that yin and yang are relative qualities that can only be determined in a comparison. All food is considered to have both properties, with one dominating. Foods with yang qualities are considered compact, dense, heavy, and hot, whereas those with yin qualities are considered expansive, light, cold, and diffuse. However, these terms are relative; "yangness" or "yinness" is only discussed in relation to other foods.

Brown rice and other whole grains such as barley, millet, oats, quinoa, spelt, rye, and teff are considered by macrobiotics to be the foods in which yin and yang are closest to being in balance. Therefore, lists of macrobiotic foods that determine a food as yin or yang generally compare them to whole grains.

Nightshade vegetables, including tomatoes, peppers, potatoes, and eggplant; also, spinach, beets, and avocados, are not recommended or are used sparingly in macrobiotic cooking, as they are considered extremely yin. Some macrobiotic practitioners also discourage the use of nightshades because of the alkaloid solanine which is thought to affect calcium balance. Some proponents of a macrobiotic diet believe that nightshade vegetables can cause inflammation and osteoporosis.

Macrobiotics was founded by George Ohsawa and popularized in the United States by his disciple Michio Kushi. In the 1960s, the earliest and most strict variant of the diet was termed the "Zen macrobiotic diet" which claimed to cure cancer, epilepsy, gonorrhea, leprosy, syphilis and many other diseases. Ohsawa wrote that dandruff is "the first step toward mental disease". Ohsawa wrote about the diet in his 1965 book Zen Macrobiotics. The Zen macrobiotic diet involved 10 restrictive stages with the highest stage eliminating all foods in the diet apart from whole grains. Fluid intake was discouraged at all stages.

In 1965, a young follower of the macrobiotic diet had died from malnutrition. George Ohsawa was sued for malpractice and the Ohsawa Foundation in New York was closed after a raid by the Food and Drug Administration. In 1966, a Grand Jury who reviewed several cases of death from malnutrition among macrobiotic proponents concluded that the diet "constitutes a public health hazard". In 1967, the first case report of scurvy on the macrobiotic diet was reported. In 1971, the American Medical Association's Council on Foods and Nutrition commented that followers of the diet were in "great danger" of malnutrition. Their report concluded that "when a diet has been shown to cause irreversible damage to health and ultimately lead to death, it should be roundly condemned as a threat to human health".