Abd al-Imam, a 27-year-old computer engineer, and two colleagues were detained a fortnight ago on charges of spreading hatred against the government and spreading false news.
"They freed all of us and now I am at home with my family," Ali Abd al-Imam, who ran www.bahrainonline.org, said.
He said he would discuss with his colleagues the fate of their website, where users often post views critical of the government and royal family.
Al-Imam had told prosecutors he could not control views expressed by visitors to the popular online forum.
The government has banned the website, which frequently changes its address to circumvent blocking by internet provider Bahrain Telecommunications Co.
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, wants more rights in the small Sunni-ruled kingdom.