Joan, often called Joan of Constantinople (c. 1199 – 5 December 1244), ruled as Countess of Flanders and Hainaut from 1205 (at the age of six) until her death. She was the elder daughter of Baldwin IX, Count of Flanders and Hainaut, and Marie of Champagne.

Orphaned during the Fourth Crusade, Joan was raised in Paris under the tutelage of King Philip II of France. He arranged her marriage to Infante Ferdinand of Portugal in 1212. Ferdinand quickly turned against Philip, starting a war that ended with the defeat of Bouvines and his imprisonment. Joan then ruled her counties alone from the age of 14. She faced the rivalry of her younger sister, Margaret, as well as the revolt of her domains – guided by a man who claimed to be her father. After the end of the war, Ferdinand was released but died soon after. Joan then married Thomas of Savoy. She died in 1244 at the Abbey of Marquette near Lille, having survived her only child, a daughter by Ferdinand.

Joan's policies favored economic development in her counties; in fact, she granted several charters to the Flemish cities. She played 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 increased, 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 was written for Joan, as well as the Life of St. Martha of Wauchier de Denain. The first novel in Dutch, Van den vos Reynaerde, was 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.

Joan's exact date of birth is unknown. Contemporary sources indicate that, like her younger sister Margaret, she was baptized in the Church of St. John of Valenciennes.

In 1202, Joan's father, Baldwin IX, left his lands to participate in the Fourth Crusade. After the capture of Constantinople, he was proclaimed emperor by the crusaders on 9 May 1204. Her mother, Marie of Champagne, decided to join him shortly after his departure, leaving their daughters Joan and Margaret in the care of their paternal uncle, Philip I of Namur. Marie decided to make a pilgrimage to the Holy Land before reuniting with her husband, but died after her arrival at Acre in August 1204. One year later, on 14 April 1205, Baldwin vanished during the Battle of Adrianople against Bulgarians and Cumans under Tsar Kaloyan of Bulgaria. His fate is unknown.

After the news of Baldwin's disappearance reached Flanders in February 1206, Joan succeeded her father as countess of Flanders and Hainaut. Because she was still a child, the administration of both counties was assumed by a council composed of the chancellor of Flanders, the provost of Lille and the castellans of Lille and Saint-Omer.