The United Arab Emirates' President Sheikh Khalifa bin Zayed Al-Nahyan died aged 73 on Friday, state media said, after battling illness for several years.
"The Ministry of Presidential Affairs condoles the people of the UAE and the Islamic world... on the passing of Sheikh Khalifa bin Zayed Al-Nahyan on Friday, May 13," the official WAM news agency said.
The ministry announced 40 days of mourning with flags at half-mast from Friday, with work suspended in the public and private sector for the first three days.
Sheikh Khalifa took over as the UAE's second president in November 2004, succeeding his father as the 16th ruler of Abu Dhabi, the federation's richest emirate.
He has rarely been seen in public since 2014, when he had surgery following a stroke, although he has continued to issue rulings.
His brother, Abu Dhabi Crown Prince Mohammed bin Zayed, has been seen as the UAE's de facto ruler in recent years.