Abdullah al-Shehni was arrested on Friday by secutiry forces, said in a statement from the Gulf Discussion Forum, which groups intellectuals of the Gulf Arab states.
His family has not heard from him and does not know where he is detained, it added.
Shehni, a former member of the UAE armed forces who was originally from the northern emirate of Ras al-Khaimah, was one of 133 signatories of a March 9 petition calling for direct elections and a parliament with legislative powers in the UAE, which has not yet seen protests that have swept other Arab states.
Three other Emirati activists have been reported arrested before him.
On April 10, Human Rights Watch denounced the arrest of Ahmed Mansoor, who was also a signatory of the petition, and called on authorities to reveal his whereabouts.
HRW said Mansoor's arrest was "aimed at scaring and intimidating others in the UAE who may wish to make public their demands for democratic reforms."
The Gulf Discussion Forum said on April 11 that economist Nasser bin Gaith had been arrested, as had another activist, Mansur al-Shehhi.
The group has said recent arrests are part of "'protests phobia' regimes in the region are suffering from."
"The repression by Bahraini authorities of movements calling for freedom and democracy is an example," said the group, adding that Oman, which has also cracked down on pro-reform protests, is another example.