Kuwait's ruling Emir Sheikh Sabah al-Ahmad al-Jaber al-Sabah has died, his office said in a statement read out on state television on Tuesday.
Sheikh Sabah, 91, had ruled the Gulf oil producer since 2006 and steered its foreign policy for more than 50 years.
"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 cabinet later declared his designated successor and brother, Crown Prince Sheikh Nawaf al-Ahmad al-Sabah, as emir. He will be sworn in on Wednesday at 11 am, said the National Assembly speaker in a tweet.
The cabinet said Sheikh Sabah had passed away on Tuesday afternoon, declaring 40 days of mourning.
The emir had been in hospital in the United States since July following surgery for an unspecified condition in Kuwait that same month.
The 15th leader in a family that has ruled for over 250 years, Sheikh Sabah weathered Kuwait's crises with shrewd judgement.
Sheikh Sabah forged very close ties with Saudi Arabia, rebuilt relations with Iraq and made fundraising for humanitarian aid in Syria one of Kuwait’s priorities.
Condolences poured in from across the Arab world, and several countries in the Gulf and wider region announced mourning periods.
“Today we lost a big brother and a wise and loving leader .. who spared no effort for Arab unity,” Jordan’s King Abdullah said in a Twitter post.
President Donald Trump earlier this month awarded the US Legion of Merit, Degree Chief Commander, to Sheikh Sabah in what the White House said was the first time the honor has been given since 1991. The emir's eldest son, Sheikh Nasser, accepted the award.
United Nations Secretary-General Antonio Guterres praised the emir as “an extraordinary symbol of wisdom and generosity, a messenger of peace, a bridge builder”.