Joanna of Flanders (1295 - 1374) was consort Duchess of Brittany by her marriage to John IV, Duke of Brittany. She was daughter of Louis, Count of Nevers and Rethel and sister of Count Louis I of Flanders.

Joanna was one of the few women in history with a military role. When her husband died in 1345 in the midst of the Breton War of Succession, she took arms to protect the rights of her son John V, Duke of Brittany against the party led by Charles of Blois and Joanna of Dreux. She organized resistance and made use of diplomatic means to protect her family. In the siege of Hennebont, she took arms and, dressed in an armour, conducted the defense of the town urging the women to cut their skirsts and take their safety in their own hands. She even led a raid of knights outside the walls that successfully destroyed one of the enemies rear camps.

Joanna died peacefully in 1374.

See also: Timeline of women's participation in warfare