King Fahd, who led Saudi Arabia for 23 years, has died and Crown Prince Abdullah was swiftly pronounced monarch of the world's largest oil exporter and key US ally.
A western diplomat said the King died in hospital today, Reuters reported.
"With deep sorrow and pain, the royal court... mourns the death of The Custodian of the Two Holy Mosques King Fahd due to illness," said an official statement read out on Saudi state television on Monday.
The announcement on television was preceded by recitations from the Quran that interrupted regular broadcasting.
Fahd, who was believed to be 83 and had been in poor health, entered hospital on 27 May with acute pneumonia.
Fahd's half-brother Crown Prince Abdullah, who has been running the kingdom's day-to-day affairs since Fahd suffered a stroke in 1995, automatically becomes king. Defence Minister Prince Sultan will be the new crown prince.
Saudi Arabia is the world's biggest oil exporter and is a close ally of the United States.
In the past two years, the kingdom has faced a violent al-Qaida campaign to end seven decades of the royal family's rule in Saudi Arabia, home to two of Islam's holiest places of worship.
The massive explosion at the port of Beirut occurred due to the detonation of 2,750 tons of ammonium nitrate, which was seized in 2014 from the ship Rhosus