Eustace Garnier (also Grenier, Granarius, Grenarius) (died June 15, 1123) was an important crusader lord, and Constable of the Kingdom of Jerusalem.
He was from Beaurain-Château, in Thérouanne in northern France (at the time in the County of Flanders); he accompanied Hugues II of St. Pol on the First Crusade, in the retinue of Godfrey of Bouillon.
While probably only a minor presence during the crusade itself, he participated in the Battle of Ramlah in 1105, and in 1109 he assisted in the siege of Tripoli. There he was one of the envoys sent by Baldwin I of Jerusalem to negotiate between William-Jordan and Bertrand of Toulouse, who disputed the claim to Tripoli. On December 19, 1111 he was granted the city of Sidon, after it was captured by Baldwin I with the help of Sigurd I of Norway. He was already lord of Caesarea, which had been captured in 1101 and given to him at an unknown date. Soon after this he married Emelota or Emma, the niece of Patriarch Arnulf of Chocques, and was also granted Jericho and its revenue, which was formerly church property. He also took part in the sieges of Shaizar, which was not captured, and Tyre (Lebanon), which was. At the siege of Tyre he supervised the construction of the siege engines. In 1120 he was present at the Council of Nablus, convened by Baldwin II, where the laws of the kingdom were first written down. When Baldwin II was captured in 1123 by the Ortoqids, Eustace was elected constable of Jerusalem and regent of the kingdom. As regent Eustace defeated an Egyptian invasion at the Battle of Yibneh on May 29, 1123. Eustace died soon after on June 15, 1123, and was replaced as Constable and Regent by William I of Bures. He was buried in Jerusalem at the Abbey of St. Maria Latina.
With Emelota he was the father of Eustace II and Walter, who succeeded him in Sidon and Caesarea respectively and were important nobles in their own right. His widow Emelota later married Hugh II of Le Puiset. According to Fulcher of Chartres he was "a stout man of honest character," and "a wise and prudent man, with great experience in military matters" according to William of Tyre.