Eleven of the defendants, whose nationalities were not disclosed, appeared at a first hearing before the Federal Supreme Court in Abu Dhabi, WAM said.
It was unclear why the remaining four were not in court.
The 15 are accused of "joining and collecting funds for two terrorist groups abroad - Al-Nusra Front and Ahrar al-Sham" which are fighting against Syrian President Bashar al-Assad's regime.
Al-Nusra is Al-Qaeda's affiliate in Syria, while Ahrar al-Sham is a powerful Islamist rebel group.
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One is accused of possessing a firearm without a permit, while another allegedly "supervised and administrated a website that disseminates information on Al-Qaeda and spreads its ideology".
The next hearing will be on September 23, it added.
The United Arab Emirates has not experienced the widespread protests that have swept other Arab states since 2011.
But authorities have cracked down hard on dissent and calls for democratic reform, drawing criticism from human rights groups.
Authorities in the Gulf country have mainly targeted Islamists with alleged links to the Muslim Brotherhood or Al-Qaeda.
Dozens of Emiratis and Egyptians have also been jailed for forming cells of the Muslim Brotherhood, which is outlawed in Egypt and accused of seeking to overthrow the Gulf monarchies.
UN expert Gabriela Knaul in March criticised "violations" and a "lack of transparency" in UAE court proceedings, and urged an independent probe into allegations of torture in prisons.