The 11 activists were arrested for marching from Songkhla to Bangkok demanding energy reforms.
Military officers arrested them for violating a law banning assembly of more than five people.
Walit Rojanapakdi, 4th Army chief, ordered the release of yesterday evening after their fellow activists agreed to cancel the march. The activists promised to be more careful about conducting their campaign in the future.
They asked permission from Lt Gen Walit to set up meetings in 14 southern provinces to inform people of their energy reform proposals.
The activists are key supporters of the Partnership for Energy Reform and set off on the long march on Tuesday. They had been held at Senanarong Army Camp in Hat Yai district.