Joan, often called Joan of Constantinople (c. 1199 – 5 December 1244), rules as Countess of Flanders and Hainaut from 1205 (at the age of six) until her death.

She is the elder daughter of Baldwin IX, Count of Flanders and Hainaut, and Marie of Champagne.

Orphaned during the Fourth Crusade, Joan is raised in Paris under the tutelage of King Philip II of France.

He arranges her marriage to Infante Ferdinand of Portugal in 1212.

Ferdinand quickly turns against Philip, starting a war that ends with the defeat of Bouvines and his imprisonment.

Joan now rules her counties alone from the age of 14.

She faces the rivalry of her younger sister, Margaret, as well as the revolt of her domains – guided by a man who claims to be her father.

After the end of the war, Ferdinand is released but dies soon after.

Joan now marries Thomas of Savoy.

She dies in 1244 at the Abbey of Marquette near Lille, having survived her only child, a daughter by Ferdinand.

Joan's policies favos economic development in her counties; in fact, she grants several charters to the Flemish cities.

She plays an important role in the development of the Mendicant orders, the Beguines, the Victorines and hospital communities in her domains (without neglecting the traditional religious orders).

Under her reign, women's foundations increase, transforming the place of women in both society and the church.

The Manessier's Continuation (also called the Third Continuation), one of the novels of the Story of the Grail, is written for Joan, as well as the Life of St. Martha of Wauchier de Denain.

The first novel in Dutch, Van den vos Reynaerde, is written by a cleric of her court.

There are several painted or sculpted representations of the Countess in France and Belgium, as well as two Géants du Nord.