Mon (紋, [mõ̞ꜜɴ]), also called monshō (紋章), mondokoro (紋所), and kamon (家紋), are Japanese emblems used to decorate and identify an individual, a family, or (more recently) an institution, municipality or business entity. While mon is an encompassing term that may refer to any such device, kamon and mondokoro refer specifically to emblems that are used to identify a family. An authoritative mon reference compiles Japan's 241 general categories of mon based on structural resemblance (a single mon may belong to multiple categories), with 5,116 distinct individual mon.

It is well acknowledged that there are a number of lost or obscure mon. Among mon, the mon officially used by the family is called jōmon (定紋). Over time, new mon have been created, such as kaemon (替紋), which is unofficially created by an individual, and onnamon (女紋), which is created by a woman after marriage by modifying part of her original family's mon, so that by 2023 there will be a total of 20,000 to 25,000 mon.

The devices are similar to the badges and coats of arms in European heraldic tradition, which likewise are used to identify individuals and families. Mon are often referred to as crests in Western literature, the crest being a European heraldic device similar to the mon in function. Japanese mon influenced Louis Vuitton's monogram designs through Japonisme in Europe in the late 1800s.

Mon originated in the mid-Heian period (c. 900–1000) as a way to identify individuals and families among the nobility. They had a pecking order, and when gissha (牛車, bullock cart) passed each other on the road, the one with the lower status had to give way, and the mon was painted on the gissha. The Heiji Monogatari Emaki, an emakimono (絵巻物, picture scroll) depicting the Heiji rebellion, shows mon painted on gissha. Gradually, the nobility began to use mon on their own costumes, and the samurai class that emerged in the late Heian period and came to power in the Kamakura period (1185–1333) also began to use mon. By the 12th century, sources give a clear indication that heraldry had been implemented as a distinguishing feature, especially for use in battle. It is seen on flags, tents, and equipment. On the battlefield, mon served as army standards, even though this usage was not universal and uniquely designed army standards were just as common as mon-based standards (cf. sashimono, uma-jirushi).

Gradually, mon spread to the lower classes, and in the Muromachi period (1336–1573), merchants painted emblems on their shop signs, which became mon. In the Edo period (1603–1867), kabuki actors used mon, and the general public was allowed to choose and use their favorite mon. By the Genroku period (1680–1709) in the early Edo period, the use of mon was fully established among the general public. However, the use of the chrysanthemum mon used by the imperial family and the hollyhock mon used by the Tokugawa clan (Tokugawa shogunate) was prohibited. Mon were also adapted by various organizations, such as merchant and artisan guilds, temples and shrines, theater troupes and even criminal gangs. In an illiterate society, they served as useful symbols for recognition.

Japanese traditional formal attire generally displays the mon of the wearer. Commoners without mon often used those of their patron or the organization they belonged to. In cases when none of those were available, they sometimes used one of the few mon which were seen as "vulgar", or invented or adapted whatever mon they wished, passing it on to their descendants. It was not uncommon for shops, and therefore shop-owners, to develop mon to identify themselves.

Occasionally, patron clans granted the use of their mon to their retainers as a reward. Similar to the granting of the patron's surnames, this was considered a very high honor. Alternatively, the patron clan may have added elements of its mon to that of its retainer, or chosen an entirely different mon for them.

Mon motifs can be broadly classified into five categories: animals, plants, nature, buildings and vehicles, and tools and patterns, each with its own meaning. The most common animal motifs are the crane and the turtle, which, according to tradition, were symbols of longevity and were used to wish the family a long and prosperous life. Plant mon were symbols of wealth and elegance, so they were often used to wish for the improvement of the family's social status and economic power, and motifs such as wisteria and paulownia were often used. Mon depicting buildings, vehicles, or tools often indicated occupation or status. For example, a mon with a torii gate indicated a family associated with Shinto, a mon with a gissha wheel indicated nobility, and a mon with a crowbar indicated a family associated with construction. The mon of nature was a symbol of respect for nature and prayers for a good harvest, and motifs such as the moon, mountains, and thunder were used.