A brassiere (pronounced UK: /ˈbræzɪər/, US: /brəˈzɪər/; commonly referred to as a bra /ˈbrɑː/) is a woman's undergarment that supports her breasts. Bras are typically form-fitting and perform a variety of functions and have also evolved into a fashion item. The primary purpose of a bra is to enhance the wearer's comfort by supporting her breasts, but women also wear bras to attract attention, to minimize attention, and for fashion reasons.

Women sometimes wear bras to conform to informal or formal social norms such as a business dress code. In Western countries, it is estimated that from 75% to 90% of women wear a bra. A minority do not wear a bra, sometimes for health or comfort reasons, or because they believe they do not need one. Some garments, such as camisoles, tank tops and backless dresses, have built-in breast support, alleviating the need to wear a separate bra.

Bra designers and manufacturers originally produced bras that were purely functional. Changing social trends and new fabrics and materials have increased the variety of available designs, and allowed manufacturers to make bras that are in some instances more fashionable than functional. Manufacturers' standards and sizes vary widely, making it difficult for women to find a bra that fits correctly. Bra-measurement methods vary, and even professional bra fitters can disagree on the correct size for the same woman. Women's breasts vary widely in size and shape; most are asymmetric to a degree and can change from month to month depending on the menstrual cycle, pregnancy, or weight gain or loss. As a result, from 75–85% of women wear an incorrect bra size.