Reginald III of Guelders (French: Rainaud or Renaud, known as "The Fat") (May 13, 1333 – December 4, 1371) was Count, later Duke, of Guelders and Count of Zutphen from 1343 to 1361, and again in 1371. He was the son of Reginald II of Guelders and of Eleanor of Woodstock, daughter of Edward II of England.
With the death of his father in 1343, his mother held the regency until 1344. From 1350, his brother Edward asserted his rights and a quarrel of succession burst out between the two brothers until 1361. Reginald was overcome in Tiel and was imprisoned in the castle of Nijenbeek. There he became so large that he could not have left, even if the door had remained open. Edward died August 24, 1371, having been mortally wounded in the Battle of Baesweiler and Reginald was released (according to the legend, the walls had to be cut so he could leave), but held the ducal throne for only a short period, because he died a few months later. He was buried at Graefenthal Abbey.
As he did not have a legitimate child, the succession passed to his sisters, who fought between them for the duchy of Guelders.
He married in Tervuren on July 1, 1347, Marie of Brabant (1325–1399), lady of Turnhout, daughter of John III, Duke of Brabant and Marie d'Evreux.
They did not have children, but Reginald had an illegitimate daughter, Ponte of Guelders, who married Johan van Groesbeek (d. 1428), lord of Heumen and Malden.
| Preceded by Reginald II |
Duke of Guelders 1343–1361 |
Succeeded by Edward |
| Preceded by Edward |
Duke of Guelders 1371–1371 |
Succeeded by War of the Succession of Guelders |