Guy of Lusignan (died 1194) was a French knight who became king of Jerusalem and led the Kingdom to disaster at the Battle of Hattin in 1187.

Guy arrived in the Kingdom in the 1170s, and in 1180 King Baldwin IV, incapacitated by leprosy, arranged for him to marry his sister Sibylla. He also became count of Jaffa and bailiff of Jerusalem, and quickly began to plot with Sibylla against Baldwin and his regent Raymond III of Tripoli. In 1182 he coerced Baldwin into naming him regent, and in 1183 Guy had Baldwin give up the crown in favour of Baldwin V, Guy's stepson. Raymond III was at first regent for Baldwin V, but Guy was able to take over this position, and when Baldwin V died in 1186, Guy claimed the throne, making himself the first Lusignan king of Jerusalem. Technically he was only the consort of Queen Sibylla, but Guy held all the actual power.

Guy came to power just as Saladin was beginning his strongest assault on the kingdom. Many of the descendants of the original crusaders, who had grown up in the kingdom, wanted to make peace with Saladin, but newcomers like Guy and Raynald of Chatillon were there to fight. Raynald had offended Saladin with his attacks on caravans even when the kingdom was at peace with the sultan, and in 1186 Saladin invaded with the intention of recapturing Jerusalem itself. In 1187 Guy attempted to relieve Saladin's siege of Tiberias, against the advice of Raymond III; Guy's army was surrounded and cut off from a supply of water, and on July 4 the army of Jerusalem was completely destroyed at the Battle of Hattin.

In the aftermath Jerusalem and most of the Kingdom fell to Saladin, except for Tyre, which was defended by another newcomer, Conrad of Montferrat. Guy lost his claim to the kingdom for good when Sibylla died in 1190. Conrad, who married Sibylla's younger half-sister Isabella, was elected in his place due to the influence of Richard I of England, who had recently arrived on the Third Crusade. Richard, who had captured Cyprus prior to his arrival, sold Guy the island in order to remove him from the internal hostilities of Palestine; Guy then became the first ruler of the Kingdom of Cyprus. Guy died in 1194, but descendants of the Lusignans continued to rule the island until 1474.

A later Guy of Lusignan (died 1344) became Constantine IV of Armenia.