Appenzell (German pronunciation: [ˈapn̩ˌtsɛl] ^ⓘ) was a canton in the northeast of Switzerland, and entirely surrounded by the canton of St. Gallen, in existence from 1403 to 1597.

Appenzell became independent of the Abbey of Saint Gall in 1403 and entered a league with the Old Swiss Confederacy in 1411, becoming a full member in 1513. It was divided into Appenzell Innerrhoden and Appenzell Ausserrhoden in 1597 (in a process called the Landteilung) as a result of the Swiss Reformation.

The territory of Appenzell as a geographical entity is known as Appenzellerland. While in political contexts, the two cantons (until 1999 half-cantons) are referred to as beide Appenzell ('both Appenzells').

The name Appenzell derives from Latin: abbatis cella 'cell (i.e., estate) of the abbot'. This refers to the Abbey of St. Gall, which exerted a great influence on the area. By the middle of the 11th century, the abbots of St Gall had established their power in the land later called Appenzell, which by that time was thoroughly Alemannic.

By about 1360, conflicts over grazing rights, taxes, and tithes were causing concern for both the abbot and the farmers of Appenzell. Both parties wanted to protect their rights and interests by joining the new Swabian League. In 1377 Appenzell was allowed to join the League with the support of the cities of Konstanz and St. Gallen (the city of St. Gallen was often at odds with the neighboring Abbey of St. Gall). With the support of the League, Appenzell refused to pay many of the gifts and tithes that the Abbot Kuno von Stoffeln demanded. In response to the loss of revenue from his estates, Kuno approached the Austrian House of Habsburg for help. In 1392 he made an agreement with the Habsburgs, which was renewed in 1402. In response, in 1401 Appenzell entered into an alliance with the city of St. Gallen to protect their rights and freedom.

Following increasing conflicts between the Appenzellers and the abbot's agents, including the bailiff of Appenzell demanding that a dead body be dug up because he wanted the man's clothes, the Appenzellers planned an uprising. On a certain day, throughout the abbot's lands, they attacked the bailiffs and drove them out of the land. Following unsuccessful negotiations, Appenzell and St. Gallen entered into a treaty. The treaty between St. Gallen and Appenzell marked a break between the abbot and his estates. Perhaps fearing the Habsburgs, in 1402 the League expelled Appenzell. During the same year, St. Gallen reached an agreement with the abbot and Appenzell could no longer count on St. Gallen's support. Appenzell declared itself ready to stand against the abbot, and in 1403 formed an alliance with the Canton of Schwyz, a member of the Old Swiss Confederation that had defeated the Austrians in the last century. Glarus provided less support, but authorized any citizen who wished to support Appenzell to do so. In response, the League raised an army and marched to St. Gallen, before heading toward Appenzell. On 15 May 1403, they entered the pass to Speicher and outside the village of Vögelinsegg met the Appenzell army. A small force of Appenzell and Confederation troops defeated the League army and signed a short lived peace treaty.

Following another Appenzell victory on 17 June 1405, at Stoss Pass on the border of Appenzell town, the new canton continued to expand. During the expansion, Appenzell had even captured the abbot of St Gall and in response they were excommunicated by the Bishop of Constance.

However, while the Bund expanded the Austrians used the peace to regain their strength. On 11 September 1406 an association of nobles formed a knightly order known as the Sankt Jörgenschild (Order of St. George's Shield) to oppose the rebellious commoners of the Bund. Following a defeat at Bregenz, Appenzell was unable to hold the Bund together. The city of St. Gallen and the Canton of Schwyz each paid off the Austrians to avoid an attack, and the Bund was dissolved by King Rupert on 4 April 1408.