Black tie is a semi-formal Western dress code for evening events, originating in British and American conventions for attire in the 19th century. In British English, the dress code is often referred to synecdochically by its principal element for men, the dinner suit or dinner jacket. In American English, the equivalent term tuxedo (or tux) is common. The dinner suit is a black, midnight blue or white two- or three-piece suit, distinguished by satin or grosgrain jacket lapels and similar stripes along the outseam of the trousers. It is worn with a white dress shirt with standing or turndown collar and link cuffs, a black bow tie, typically an evening waistcoat or a cummerbund, and black patent leather dress shoes or court pumps. Accessories may include a semi-formal homburg, bowler, or boater hat. For women, an evening gown or other fashionable evening attire may be worn.

The first dinner jacket is traditionally traced to 1865 on the then Prince of Wales, later King Edward VII (1841–1910). The late 19th century saw gradual introduction of the lounge jacket without tails as a less formal and more comfortable leisure alternative to the frock coat. Similarly, the shorter dinner jacket evolved as a less formal alternative to the dress coat out of the informal smoking jacket, itself derived from out of the banyan. Thus in many non-English languages, a dinner jacket is still known as the false friend "smoking". In American English, its synonym "tuxedo" was derived from the village of Tuxedo Park in New York State, where it was introduced in 1886 following the example of Europeans. Following the counterculture of the 1960s, black tie has increasingly replaced white tie for more formal settings in the United States, along with cultures influenced by American culture.

Traditionally worn only for events after 6 p.m., black tie is less formal than white tie but more formal than informal or business dress. As semi-formal, black tie are worn for dinner parties (public, fraternities, private) and sometimes even to balls and weddings, although etiquette experts discourage wearing of black tie for weddings. Traditional semi-formal day wear equivalent is black lounge suit. Supplementary semi-formal alternatives may be accepted for black tie: mess dress uniform, religious clothing (such as cassock), folk costumes (such as highland dress), etc.