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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. The word "astringent" derives from Latin astringere, meaning "to bind fast". Two common examples are calamine lotion and witch hazel. The proprietary product Sea Breeze, which is an aqueous solution of denatured alcohol, camphor (0.68%) and several aromatic essential oils, is an astringent that is often used as an aftershave.

Astringency is also the dry, mouth-puckering sensation caused by tannins found in many fruits such as blackthorn, bird cherry and persimmon fruits. The tannins denature the salivary proteins, causing a rough "sandpapery" sensation in the mouth. Astringency tastes unpleasant to 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 survived in some plant varieties because birds are better than mammals at long-distance seed dispersal, often flying some distance before passing the seeds in their droppings, so plants that contained astringent substances in the fruit are more successful than those that did not.

Astringent substances are also found in some wines and teas. A small amount of astringency is expected in some wines, especially young red wines made from grapes such as cabernet sauvignon.

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 worsen the clogging of pores^{[citation needed]}. Astringents also help heal stretch marks and other scars^{[citation needed]}. 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^{[citation needed]}.

Some common astringent agents include alum, oatmeal, witch hazel, very cold water, and rubbing alcohol eg Surgical Spirit. Astringent preparations include silver nitrate, zinc oxide, zinc sulfate, 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.