The Sultan of Oman, Qaboos bin Said al-Said, died on Friday evening, Oman state TV and the state news agency Twitter account said on early Saturday, Reuters reported.
Qaboos, 79, had ruled the Gulf Arab state since he took over in a bloodless coup against his father in 1970 with the help of the British.
The ruler of Oman did not have any children and did not publicly appointed a successor. According to a statute from 1996, the country's ruling family must choose a successor within three days.
A three-day mourning period has been declared, and flags are to be flown at half mast for 40 days, Reuters reported, citing state media.