The ruler of Ras Al Khaimah, the UAE's fourth largest emirate, died on Wednesday after several months of illness, the state news agency said.
His Highness Sheikh Saqr Al Qasimi will be succeeded by his son, the Crown Prince Sheikh Saud bin Saqr Al Qasimi.
His Highness Sheikh Saqr Al Qasimi, believed to be in his 90s, ruled the northern emirate of Ras Al Khaimah for more than 60 years. Sheikh Saqr was the world's longest serving monarch, having taken office in 1948, and the last surviving ruler to have been in power at the creation of the UAE in 1971.
He had been in hospital for several months.
His son, His Highness Sheikh Saud bin Saqr Al Qasimi, was appointed Crown Prince and Deputy Ruler in June 2003.
Official mourning has been declared with effect from Wednesday for a week, through which flags will be flown at half mast. Government ministries and institutions will be closed.
Ras Al Khaimah sits on the Strait of Hormuz, through which 40 percent of the world's seaborne oil passes.
The emirate's economy is based on industries like cement, pharmaceuticals, and glass, and building a regional manufacturing and shipping hub. Its gross domestic product accounts for 1.5 percent of the UAE's economy.
The emirate has its own sovereign wealth fund, the Ras Al Khaimah Investment Authority (RAKIA), which has a portfolio of around $2bn.