A masquerade ball (or bal masqué) is an event which the participants attend in costume wearing a mask. (A masque is a formal written and sung court pageant.)

Such gatherings, festivities of Carnival, were paralleled from the 15th century by increasingly elaborate allegorical Entries, pageants and triumphal processions celebrating marriages and other dynastic events of late medieval court life. Masquerade balls were extended into costumed public festivities in Italy during the 16th century Renaissance (Italian, maschera). They were generally elaborate dances held for members of the upper classes, and were particularly popular in Venice. They have been associated with the tradition of the Venetian Carnival. With the fall of the Venetian Republic at the end of the 18th century, the use and tradition of masks gradually began to decline, until they disappeared altogether.
They became popular throughout mainland Europe in the 17th and 18th centuries, sometimes with fatal results. Gustav III of Sweden was assassinated at a masquerade ball by disgruntled nobleman Jacob Johan Anckarström, an event which Eugène Scribe and Daniel Auber turned into the opera Gustave III. The same event was the basis of Giuseppe Verdi's opera A Masked Ball, although the censors in the original production forced him to portray it as a fictional story set in Boston. Most came from countries like Switzerland and Italy.

The "Bal des Ardents" ("Burning Men's Ball") was intended as a Bal des sauvages ("Wild Men's Ball") a costumed ball (morisco). It was in celebration of the marriage of a lady-in-waiting of Charles VI of France's queen in Paris on January 28, 1393. The King and five courtiers dressed as wildmen of the woods (woodwoses), with costumes of flax and pitch. When they came too close to a torch, the dancers caught fire. (This episode may have influenced Edgar Allan Poe's short story "Hop-Frog".) Such costumed dances were a special luxury of the ducal court of Burgundy.
John James Heidegger, a Swiss count, is credited with having introduced the Venetian fashion of a semi-public masquerade ball, to which one might subscribe, to London in the early eighteenth century, with the first being held at Haymarket Opera House. Throughout the century the dances became popular, both in England and Colonial America. Its prominence did not go unchallenged; a significant anti-masquerade movement grew alongside the balls themselves. The anti-masquerade writers (among them such notables as Samuel Richardson) held that the events encouraged immorality and "foreign influence". While they were sometimes able to persuade authorities to their views, enforcement of measures designed to end masquerades was at best desultory.

Masquerade balls were sometimes set as a game among the guests. The masked guests were supposedly dressed so as to be unidentifiable. This would create a type of game to see if a guest could determine each others' identities. This added a humorous effect to many masques and enabled a more enjoyable version of typical balls.

One of the most noted masquerade balls of the 20th century was that held at Palazzo Labia in Venice on 3 September 1951, hosted by Carlos de Beistegui. It was dubbed "the party of the century".

A new resurgence of masquerade balls began in the late 1990s in North America and are still held today, though in modern times the party atmosphere is emphasized and the formal dancing usually less prominent. Less formal "costume parties" may be a descendant of this tradition.

The picturesque quality of the masquerade ball has made it a favorite topic or setting in literature. Edgar Allan Poe's short story "The Masque of the Red Death" is based at a masquerade ball in which a central figure turns out to be exactly what he is costumed as.
Another ball in Zürich is featured in the novel Steppenwolf by Hermann Hesse.

"Regency" romance novels, which are typically about Britain's upper class "ton" during the 1800s, often make use of masquerade balls as settings, due both to their popularity at the time and to their endless supply of plot devices.

As mentioned before, Masquerades are the centers of multiple operas. The musical and movie The Phantom of the Opera has a very important scene in the story line take place at a masked ball. This scene (in the film) features inventive choreography and music by Andrew Lloyd Webber.