A tea cosy or tea warmer is a cover for a teapot, traditionally made of cloth. It insulates a teapot, keeping the contents warm. Their use predates the invention of vacuum flasks as a means of keeping hot liquids hot.

Sometimes, if the tea is served in a restaurant or in a hotel, the teapot is covered with a tea cosy that has a metal exterior to protect the inner fabric of the cosy from wear and tear and also to further improve the insulation of the teapot. A typical cosy is easy to put over or pop off the teapot in order to pour the tea, but some are wrapped around the teapot and have holes for the spout and the handle (so called "bachelor" teapots). The "crinoline lady" cosies include a porcelain doll on the top, with her flowing skirts providing the thermal insulation.

Tea cosies may have padded inserts that can be removed and washed. Some tea cosies are hand-knitted, resembling woollen hats, some even feature a "bobble" (pom-pom) on top, which may also serve as a handle to remove or lift the tea cosy. A special tea cosy is the so-called tea lugger, which enables the hot teapot to be carried around easily.

Tea cosies may sometimes be made in matching sets with items such as tablecloths, place mats, egg cosies, oven gloves, or aprons. Cloth tea-cosies may be embroidered, perhaps to complement a fine set of china. Some have been made with hidden pockets to be filled with fragrant herbs or flowers, similar to a potpourri.