High Court Judge Samuel Kudya ruled Wednesday that state prosecutors failed to show there was a threat to state security and had "bald and unsubstantiated" evidence against the six, including former opposition lawmaker Munyaradzi Gwisai.
Treason carries a possible death sentence in Zimbabwe. Gwisai and his co-accused were ordered to pay a $2,000 bail.
The group was arrested Feb. 19 for attending a meeting that showed footage of uprisings in Tunisia and Egypt. They deny any wrongdoing, saying the meeting was an academic lecture on democratic rights.
The six are scheduled to return to court March 21.