Kuwait's Emir, Sheikh Sabah al-Ahmed al-Sabah, has died at the age of 91, the state media reported.
He was expected to be succeeded by his 83-year-old half-brother and crown prince, Sheikh Nawaf al-Ahmed. reports the BBC.
In July, Sheikh Sabah was flown to the United States for medical treatment following surgery for an unspecified condition in Kuwait.
He had ruled the oil-rich Gulf Arab state since 2006 and had overseen its foreign policy for more than 50 years.
He was dubbed the "dean of Arab diplomacy" for his efforts to restore relations with states that backed Iraq during the 1990-1991 Gulf War, when Kuwait was invaded by Iraqi forces.
The emir also often acted as a mediator in regional disputes, including the ongoing diplomatic stand-off between Saudi Arabia, its allies and Qatar.
Kuwait also refrained from intervening in Syria's civil war, instead hosting several donor conferences for humanitarian aid.