Joan of Penthièvre or Joan the Lame (in French Jeanne de Penthièvre, Jeanne la Boiteuse) (c.1319 – September 10, 1384) reignsas Duchess of Brittany suo jure together with her husband Charles of Blois between 1341 and 1364.

Her ducal claims are contested by the House of Montfort, which prevaila only after an extensive civil war, the War of the Breton Succession.

After the war, Joan remains titular Duchess of Brittany to her death.

She is Countess of Penthièvre in her own right throughout her life.