The restoration of ties comes amid a broader regional push for reconciliation and more than two years after Arab states ended a boycott of Doha that had shattered the Western-allied Gulf Arab bloc.
The Qatari ministry said in a statement that the two foreign ministers, Sheikh Mohammed bin Abdulrahman Al-Thani and Sheikh Abdullah bin Zayed, had held a phone call as the two embassies reopened on Monday.
Abu Dhabi and Doha had been locked for years in a bitter rivalry over regional influence, the role of Islam in politics and over support for pro-democracy movements across the Middle East.
In 2017, Saudi Arabia, the UAE, Bahrain and Egypt severed all ties with Qatar over accusations that it supported terrorism, a reference to political Islam, and that it had forged closer ties with Iran. Doha denied the charges.
The restoration of ties also came after Iran and Saudi Arabia agreed to re-establish relations after years of hostility, which had threatened instability in the Gulf and stoked the war in Yemen.