Countess Joan of Hainaut (c. 1320–1386) was a noblewoman from the region of Hainaut in present-day Belgium. She was the daughter of Count William I of Hainaut and Countess Joan of Rethel. Joan became the Countess of Hainaut and Holland through her marriage to Count William III of Holland, strengthening political ties in the region. Joan played a significant role in the governance of her territories, especially during her husband's absence. She was known for her efforts to maintain peace and stability in her lands, contributing to the development of Hainaut and Holland during the late medieval period.