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An astringent substance is a chemical substance that tends to shrink or constrict body tissues, usually locally after topical medicinal application. Two common examples are calamine lotion and witch hazel.

Astringent substances are also found in many fruits such as blackthorn, bird cherry and persimmon fruits, giving them their mouth-puckering qualities. Astringency is tasted as unpleasant by many mammals (including humans), which tend to avoid eating astringent fruit; conversely, birds do not taste astringency and readily eat these fruit. It is thought that fruit astringency evolved in plants because birds are better at long-distance seed dispersal (often flying some distance before passing the seeds in their droppings) than mammals, so plants that contained astringent substances in the fruit are more successful than those that did not.

Astringent medicines cause shrinkage of mucous membranes or exposed tissues and are often used internally to check discharge of blood serum or mucous secretions. This can happen with a sore throat, hemorrhages, diarrhea, or with peptic ulcers. Externally applied astringents, which cause mild coagulation of skin proteins, dry, harden, and protect the skin. Acne sufferers are often advised to avoid astringents, which are believed to aid in clogging pores. Mildly astringent solutions are used in the relief of such minor skin irritations as those resulting from superficial cuts, allergies, insect bites, or fungal infections such as athlete's foot.

Some common astringent agents include alum, oatmeal, very cold water, and rubbing alcohol. Astringent preparations include silver nitrate, zinc oxide, Burow's solution, tincture of benzoin, and vegetable substances such as tannic and gallic acids. Some metal salts and acids have also been used as astringents.

The word "astringent" is the present participle of the Latin word astringere, meaning "to bind fast".