Joan, Countess of Flanders and Hainault (born 1199 or 1200)

Joan, often called Joan of Constantinople, ruled as Countess of Flanders and Hainaut from 1205 until her death. She was the elder daughter of Baldwin IX, Count of Flanders and Hainaut, and Marie of Champagne.

The County of Hainaut, sometimes spelled Hainault, was a territorial lordship within the medieval Holy Roman Empire that straddled the present-day border of Belgium and France. Its most important towns included Mons, now in Belgium, and Valenciennes, now in France.