Isabelle of Luxembourg (1247–1298) was a countess consort of Flanders and a marquis consort of Namur.
She was the daughter of Henry V of Luxembourg and his wife, Margaret of Bar. Isabelle was a member of the House of Luxembourg. Isabelle was the third of seven children. Her elder brother was Henry VI, Count of Luxembourg, successor to their father. Her sister was Philippa of Luxembourg, who married John II, Count of Holland and were parents to twelve children. Philippa of Hainault, Queen consort of Edward III of England was a descendant of Philippa. Her maternal grandparents were Henry II of Bar and his wife Philippa of Dreux, and her paternal grandparents were Waleran III of Limburg and his wife Ermesinde, Countess of Luxembourg.
In March 1265, Isabelle married Guy of Dampierre. Her marriage was determined by events that occurred many years before her birth. Indeed, around 1165, her great-grandfather Henry IV the Blind, Count of Namur and Luxembourg, had no children from his first marriage. He named his brother Baldwin IV of Hainaut as his successor. Baldwin died in 1171, and Henry the Blind confirmed his nephew Baldwin V of Hainaut. But with one more attempt to have children, Henry the blind married his second wife, Agnes of Gelderland, who bore a daughter, Ermesinde and thus had broken the promise he had made to Baldwin. A war ensued, with the result that Baldwin would be Henry's designated heir in Namur.
Isabelle's father claimed his rights of Namur, being son of Ermesinde. Henry V failed on his claim to rule Namur. Isabelle's parents wanted to make peace with Guy over a dispute of Namur. Isabelle became Guy's second wife; his first wife, Matilda had died one year earlier. Guy already had eight children with Matilda. Isabelle gave birth to another eight children:
- Beatrix (d. 1307), married c. 1287 Hugh II of Châtillon
- Margaret (d. 1331), married on 14 November 1282 at Roxburgh Alexander of Scotland (son of Alexander III of Scotland), married on 3 July 1286 in Namur Reginald I of Guelders
- Isabelle (d. 1323), married 1307 Jean de Fiennes, Lord of Tingry and Chatelain of Bourbourg. Mother of Robert de Fiennes, Constable of France.
- Philippa (d. 1306, Paris)
- John I, Marquis of Namur (1267–1330)
- Guy of Namur (d. 1311), Lord of Ronse, sometime Count of Zeeland
- Henry (d. 6 November 1337), Count of Lodi, married January 1309 Margaret of Cleves and had issue
- Jeanne (d. 1296), a nun at Flines
Guy arranged a marriage between their daughter Philippa and Edward, Prince of Wales. However, Philip IV of France imprisoned Guy and two of his sons, forced him to call off the marriage, and imprisoned Philippa in Paris until her death in 1306. Guy was summoned before the king again in 1296, and the principal cities of Flanders were taken under royal protection until Guy paid an indemnity and surrendered his territories, to hold them at the grace of the king.
Isabelle died in the September 1298, and her husband six years later, in 1304.