John of Brienne (c. 1170 – 19–23 March 1237) was the king of Jerusalem from 1210 to 1225 and Latin emperor of Constantinople from 1229 to 1237. He was the youngest son of Erard II of Brienne, a wealthy nobleman in Champagne. John, originally destined for an ecclesiastical career, became a knight and owned small estates in Champagne around 1200. After the death of his brother Walter III, he ruled the County of Brienne on behalf of his minor nephew Walter IV, who lived in Italy.
The barons of the Kingdom of Jerusalem proposed that John marry their queen, Maria. With the consent of Philip II of France and Pope Innocent III, he left France for the Holy Land and married Queen Maria; the couple were crowned in 1210. After Maria's death in 1212 John administered the kingdom as regent for their infant daughter Isabella II; Maria's influential uncle, John of Ibelin, attempted to depose him. King John was a leader of the Fifth Crusade. Although his claim of supreme command of the crusader army was never unanimously acknowledged, his right to rule Damietta (in Egypt) was confirmed shortly after the city fell to the crusaders in 1219. He claimed the Armenian Kingdom of Cilicia on behalf of his second wife, Stephanie, in 1220. After Stephanie and their infant son died that year, John returned to Egypt. The Fifth Crusade ended in failure (including the recovery of Damietta by the Egyptians) in 1221.
