Sheikh Nawaf al-Ahmad Al-Sabah was Kuwait's emir for just three years but spent decades in senior posts through the House of Sabah's tumultuous rule of the state.
He was succeeded by his half-brother, Sheikh Meshal, who takes over the oil-rich Gulf kingdom at the age of 83, state television reported.
Sheikh Meshal who had already assumed some key constitutional duties of the emir in 2021- has spent most of his career in Kuwait's security and intelligence apparatus, keeping a distance from the often bitter disputes within the royal family.
His predecessor's reign was short but tumultuous, marked by deep political and economic challenges triggered by a fall in oil prices that have left it lagging its wealthy Gulf neighbours, the United Arab Emirates, Qatar and Saudi Arabia.
State TV cut its regular programming and switched to a broadcast of a Quran recital before announcing Sheikh Nawaf's death.
"With great sadness and sorrow, we mourn... the death of Sheikh Nawaf al-Ahmad Al-Sabah, Emir of the State of Kuwait," said a statement on state TV.
Public offices will shut for three days and 40 days of mourning were declared.
Sheikh Nawaf, who will be laid to rest on Sunday, had been hospitalised in November for an undisclosed "emergency", but had been said to be in a stable condition.
'Devout, modest'
Born in 1937, Sheikh Nawaf took over as emir in September 2020 upon the death of another half-brother, Sheikh Sabah, at the age of 91.
The current crown prince, Sheikh Mishal al-Ahmad al-Sabah, also a half-brother, is 83 and much attention will now be focused on whether a younger ruler is brought in by the family.
Sheikh Nawaf was the fifth son of Kuwait's late ruler from 1921 to 1950 Sheikh Ahmad al-Jaber Al-Sabah.
He received secondary schooling in Kuwait but did not go on to higher education.
He started his political career at 25 as governor of Hawalli province, where he remained until 1978, before taking on the role of interior minister for a decade.
Sheikh Nawaf showed a rare sign of public emotion when he was formally sworn in by the Gulf state's Gulf state's national assembly in 2020.
Kuwait, a conservative country where sovereign powers are concentrated in the hands of the ruling Al Sabah family, is home to the most active and powerful parliament in the Gulf.
But repeated standoffs between elected lawmakers and cabinet ministers installed by the ruling family have hindered development efforts and discouraged investors.
Sheikh Nawaf's rule saw three parliamentary elections in as many years, and there have been five different government cabinets in 2023.
The political deadlock has delayed necessary reforms and blocked development projects, leaving infrastructure and education in disrepair and much of the population disgruntled.
Sheikh Nawaf issued numerous amnesties for political prisoners, earning him the title "emir of pardons".
He will "be remembered for his unique personal attributes: soft spoken, devout, modest, low profile", said Bader al-Saif, a history professor at Kuwait University.
As Sheikh Meshal becomes Kuwait's 17th ruler, the state will have a year to name a new crown prince amid speculation on whether he will appoint an heir from a younger generation.
He was deputy chief of the Kuwait National Guard from 2004 to 2020, and was named crown prince when Sheikh Nawaf took power.
The choice of crown prince will reflect "internal ruling family dynamics," said al-Saif.
"Whether Kuwait witnesses its first generational shift in decades or not... the country will need to deliver on various fronts."
Sheikh Nawaf became defence minister two years before the start of the seven-month Iraqi occupation in 1991.
After the liberation by a US-led international force, he was named minister for social affairs and excluded from the government formed after the first post-war election in 1992.
Sheikh Nawaf returned as deputy commander of the national guard in 1994 and in 2003 became interior minister again.
This period was marked by a series of deadly clashes between Kuwaiti security forces and Islamic militants in January 2005.
On becoming emir, Sheikh Nawaf had to steer the economy through a crisis caused by a fall in oil prices that saw Kuwait's credit rating cut by international agencies in 2020.
He acknowledged the "serious" challenges in his inauguration speech and the government spent heavily, doubling public debt in 18 months, to guide the state through the Covid-19 pandemic.
He made few dramatic changes, however.
Kuwait has maintained its hardline stance on Israel even as Bahrain and the United Arab Emirates have established relations. It remains a diplomatic rarity in maintaining close relations with both Saudi Arabia and its regional rival Iran.
Mohammed al-Faily, an expert in constitutional law at Kuwait University, said Sheikh Nawaf was generally considered "a calm person who, when it calls for it, can take firm decisions".
The future for the royal family remains uncertain.
There are bitter divisions within the Sabah family, with lurid accusations of corruption and political conspiracies lodged by some members against rivals.
Kuwait's constitution stipulates only that the ruler should be a descendant of the nation's founder, Mubarak Al-Sabah. By tradition, the throne had alternated between the Salem and Jaber branches of the family.
But that pattern has been broken this decade, with Sheikh Sabah, Sheikh Nawaf and the heir-apparent Sheikh Mishal all from the Jaber clan.