The Duchy of Guelders (Dutch: Gelre; French: Gueldre; German: Geldern) is a historical duchy, previously county, of the Holy Roman Empire, located in the Low Countries.

The duchy was named after the town of Geldern (Gelder) in present-day Germany. Though the present province of Gelderland (English also Guelders) in the Netherlands occupies most of the area, the former duchy also comprised parts of the present Dutch province of Limburg as well as those territories in the present-day German state of North Rhine-Westphalia that were acquired by Prussia in 1713, which included the duchy's capital Geldern.

Four parts of the duchy had their own centres, as rivers separated them:

spatially separated from the Lower Quarters (Gelderland):

The county emerged about 1096 when the first documented reference to Gerard III of Wassenberg as "Count of Guelders" occurred. It was then located on the territory of Lower Lorraine, in the area of Geldern and Roermond, with its main stronghold at Montfort (built 1260). Count Gerard's son Gerard II in 1127 acquired the County of Zutphen in northern Hamaland by marriage. In the 12th and 13th centuries, Guelders quickly expanded downstream along the sides of the Maas, Rhine, and IJssel rivers and even claimed the succession in the Duchy of Limburg, until it lost the 1288 Battle of Worringen against Berg and Brabant.

Guelders was often at war with its neighbours, not only with Brabant but also with the County of Holland and the Bishopric of Utrecht. However, its territory grew not only because of its success in warfare but also because it thrived in times of peace. For example, the larger part of the Veluwe and the city of Nijmegen were given as collateral to Guelders by their cash-strapped rulers. On separate occasions, in return for loans from the treasury of Guelders, the bishop of Utrecht granted the taxation and administration of the Veluwe, and William II – Count of both Holland and Zeeland, and who was elected anti-king of the Holy Roman Empire (1248–1256) – similarly granted the same rights over Nijmegen; as neither ruler proved able to repay their debts, these lands became integral parts of Guelders.^{[citation needed]}

In 1339, the Emperor Louis IV of Wittelsbach elevated Count Reginald II of Guelders (also styled Rainald), of the House of Wassenberg, to the rank of Duke. After the Wassenberg line became extinct in 1371 following the deaths of Reginald II's childless sons Edward II (on 24 August, from wounds suffered in the Battle of Baesweiler) and Reginald III (on 4 December), the ensuing Guelders War of Succession (1371 to 1379) saw William I of Jülich emerge victorious. William was confirmed in the inheritance of Guelders in 1379, and from 1393 onwards held both duchies in personal union (in Guelders as William I, and Jülich as William III).

In 1423, Guelders passed to the House of Egmond, which gained recognition of its title from Emperor Sigismund of Luxembourg but was unable to escape the political strife and internecine conflict that had so plagued the preceding House of Jülich-Hengebach, and more especially, the pressure brought to bear by the expansionist rulers of the Duchy of Burgundy. The first Egmond Duke, Arnold, suffered the rebellion of his son Adolf and was imprisoned by the latter in 1465. Adolf, who had enjoyed the support of Burgundian Duke Philip III ("the Good") and of the four major cities of Guelders during his rebellion, was unwilling to strike a compromise with his father when this was demanded by Philip's successor, Duke Charles the Bold. Charles had Duke Adolf captured and imprisoned in 1471 and reinstated Arnold on the throne of the Duchy of Guelders. Charles then bought the reversion (i.e., the right of succession to the throne) from Duke Arnold, who, against the will of the towns and the law of the land, pledged his duchy to Charles for 300,000 Rhenish florins. The bargain was completed in 1472–73. Upon Arnold's death in 1473, Duke Charles added Guelders to the "Low Countries" portion of his Valois Duchy of Burgundy.