Saudi King Abdullah bin Abdulaziz has died in hospital, BBC reported quoting the state TV of Saudi Arabia.
The announcement was made early on Friday, said his brother, Salman, who replaced Abdullah as the new ruler.
Before the announcement, Saudi television cut to Koranic verses, which often signifies the death of a senior royal.
King Abdullah, who was said to be aged about 90, had been in hospital for several weeks suffering from a lung infection.
Abdullah came to the throne in 2005 but had suffered frequent bouts of ill health in recent years.
King Salman, 79, had recently taken on the ailing monarch's responsibilities.
The late king's half brother Moqren has been named the new crown prince, the official statement said.
King Salman called on the royal family's Allegiance Council to recognise Moqren as his crown prince and heir.
"His Highness Salman bin Abdulaziz Al Saud and all members of the family and the nation mourn the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz, who passed away at exactly 1am this morning," the statement said.
John Kerry expresses sorrow
US Secretary of State John Kerry has expressed profound shock at the death of the Saudi king.
In a message, he stated: "This is a sad day. The United States has lost a friend, and the Kingdom of Saudi Arabia, the Middle East, and the world has lost a revered leader."
"King Abdullah was a man of wisdom and vision. I loved my visits with him as a Senator and as Secretary. Even as he battled age and illness, he held on to his sense of determination. His stories of his father and of his family were remarkable. He was so proud of the Kingdom’s journey, a brave partner in fighting violent extremism who proved just as important as a proponent of peace."
The courageous Arab Peace Initiative that he sponsored remains a critical document for the goal we shared of two states, Israel and Palestine, living in peace and security. He also made great strides to invest in the Saudi people, and the Kingdom’s infrastructure and economic development," he added.
He further stated: "The scholarship program that bears his name represents an enormous, far-sighted contribution to Saudi Arabia’s future prosperity. I know he was very proud of his role as Custodian of the Two Holy Mosques and as leader of his people."
"Teresa and I send our condolences to the family of King Abdullah and to the people of Saudi Arabia and the region."