Sheik Sabah, 91, had ruled the wealthy oil producer and U.S. ally since 2006 and steered its foreign policy for more than 50 years. His designated successor is his brother, Crown Prince Sheikh Nawaf al-Ahmad al-Sabah.
"With the utmost sadness and grief for the Kuwaiti people, the Islamic and Arab world and people of friendly nations, the Emiri Diwan mourns the death of Sheikh Sabah al-Ahmad al-Jaber al-Sabah, the emir of Kuwait," his office said.
The emir had been in hospital in the United States since July following surgery for an unspecified condition in Kuwait that same month.
INTERPRETER: It is with great sadness and sorrow the emir's court sends its condolences to the Kuwaiti people, as well as the Arab and Islamic world, and the people of friendly nations, the passing away of, the forgiven, god willing, his highness Sheikh Sabah al-Ahmad al-Jaber al-Sabah, the emir of Kuwait, who is now next to god.
Praying to god that he has mercy on him, and places him in heaven. And there is no power but from God. To god we belong, and to God we return.