They said 27-year-old Ali Abd al-Imam, who runs www.bahrainonline.org, was held for allegedly stirring hatred against the government and spreading false news that could jeopardise state security.
Officials were not immediately available to comment on Monday.
The website, a forum where users often post views critical of the government and the royal family, has been banned by the government.
It is blocked by internet provider Bahrain Telecommunications Co, but it frequently changes its web address to circumvent this.
Bahrain, the Gulf's banking hub and home to the US Navy's Fifth Fleet, has introduced some reforms, but the opposition, led by the country's majority Shia Muslims, want more rights in the Sunni-ruled island state.