The Crown Prince of Saudi Arabia, Sultan bin Abdul-Aziz, has died in New York.
The 86-year-old was both heir to the throne, and the country's defence and aviation minister.
It is understood he was suffering from colon cancer and had undergone several operations in the United States in recent years.
The royal court confirmed the death in a statement carried by the state news agency.
"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," it said.
Crown Prince Sultan was a member of the most powerful family group in Saudi Arabia, the Sudairi Seven, and one of the sons of the country's founder, King Abdulaziz, known as Ibn Saud.
He oversaw extraordinary expenditure on modernising the armed forces - with multi-billion dollar deals making Saudi Arabia one of the world's biggest arms spenders.
Prince Sultan was also involved in the setting up and development of the national airline, Saudia.
He was one of the strongest supporters of forging close ties with the US, which faced its biggest challenge after the September 2001 attacks.