Sultan bin Abdulaziz al-Saud had been in the US for medical treatment since June
Saudi Arabia's Crown Prince Sultan bin Abdulaziz al-Saud died at dawn on Saturday abroad, the country's royal court said in a statement carried by state media.
Sultan, who was thought to be aged about 86, had been in the United States for medical treatment since June.
As well as heir to the throne of the world's top oil exporter, he had been defence minister and minister of aviation for about four decades.
"With deep sorrow and sadness the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz mourns the death of his brother and his Crown Prince Sultan... who died at dawn this morning Saturday outside the kingdom following an illness," said the statement carried on state news agency SPA and state television.
Saudi television broke its schedules early on Saturday to broadcast Koranic verses accompanied by footage of the Kaaba in Mecca, Islam's holiest site.
Funeral services will be held in the Saudi capital Riyadh on Tuesday, SPA said.
Widely thought to be next in line after Sultan is Interior Minister Prince Nayef, who was named second-deputy prime minister in 2009.
King Abdullah is in his late 80s and underwent back surgery earlier this month but has been pictured since then in apparently good health.
The king was absent for three months late in 2010 while he underwent treatment for a herniated disc that caused blood to accumulate around his spine.
Prince Nayef, who is in his late 70s, has a reputation as being more conservative than either the crown prince or king.
Unlike in European monarchies, the line of succession does not move directly from father to eldest son, but has moved down a line of brothers born to the kingdoms's founder Ibn Saud, who died in 1953.
May his soul rest in peace.
MAY HIS REST IN PEACE,ITS VERY SAD TO HEAR THIS NEWS
A charity and giving person, the crown prince has many of his presences in the world of poverty as charity giver to the saudis and non saudis inside and outside saudi arabia. may the old mighty bless your soul your higness and may God bless our king Abdullah and give him health and long life. God bless our royal family and protect them and saudi arabia from the villians
May Jannatul Firdaus become his final abode. And may Allah (S.W.T) grant us the fortitude to bear the loss.
Very sad. May his soul rest in peace and good blessings, health and spirits to the family.
May his soul rest in peace, ans Allah almighty shower his blessing on him.We all are with the people of Saudi arabia on his sad demis.
Wold is deprived from a soft hearted and kind person.I convey my heartful feeling to the Royal family.