A cocktail dress or cocktail gown is a shorter-length lady's gown worn at cocktail parties and other semi-formal and formal occasions. Less elaborate versions may be worn at semi-formal occasions when gentlemen are in dark suits. (When gentlemen are in business suits, ladies also wear dress suits or "good" afternoon dresses). A longer, full-skirted cocktail dress may accompany black tie (although in the late 1960s and early 1970s short cocktail dresses were acceptable black tie attire).

In the mid-20th Century, some couturiers began describing full-skirted cocktail gowns as 'dancing costumes'. These are suitable for formal dances but not for balls, or other white tie affairs, where ball gowns must be worn.

The length of cocktail dresses ranges from just below the knee to tea length (about two inches above the ankle) and sometimes ballerina length (touching the ankle), although ballerina length dresses usually fall into the evening gown category.

A wide variety of materials, styles and accessories are acceptable; silk, satin and chiffon are always popular choices.