Abdul-Rahman al-Nuaimi was released Saturday, several Arabic-language newspapers reported.
Police detained al-Nuaimi, who taught Islamic studies at Qatar University, in June 1998 after he wrote an open letter to Qatar's Shura, or consultative council, in which he complained that government policies violated Islamic principles. The council advises the emir.
The letter, which was published in several local newspapers, criticized the sale of alcohol and the empowerment of women, who can hold government jobs, vote, and run for office in Qatar.
"This leads to an unIslamic mingling of the sexes and to women losing their proper role and turning into men," said the letter, which was signed by 12 other men, including three from the ruling Al Thani family.
Qatari officials could not be reached for comment on the release. Al-Nuaimi also could not be reached.
Al-Nuaimi was never formally charged or put on trial. His detention led to protests in Britain by Islamic activists who mounted a campaign for his release.
Qatar's emir, Sheik Hamad bin Khalifa Al Thani, has initiated a number of political reforms since he seized power in 1995.
The emirate now boasts the region's most open press. In March 1999, Qatar held elections in which women took full part, becoming one of only three Gulf states to grant political rights to women.
An increasing number of women have entered the government. In 1998, the emir appointed the country's first woman deputy minister.