A court in Bangkok freed the pair because there were no witnesses to the arson attack on Central World, Thailand's biggest shopping mall which was gutted by the blaze.
Saichon Paebua -- one of the Red Shirts' own security guards -- and co-defendant Pinit Chanarong both denied the charges.
Central World was one of dozens of buildings torched after a crackdown by armed troops firing live rounds and backed by armoured vehicles brought an end to the two months of demonstrations on May 19, 2010.
The Red Shirts -- who are broadly loyal to fugitive former premier Thaksin Shinawatra -- have always denied responsibility for the fire.
"Today's ruling proves that Red Shirt protesters did not burn down Central World," said one of their leaders, Weng Tojirakarn.
The two defendants were freed as they have already served jail terms for violating an emergency decree imposed by the authorities to deal with the protests, which paralysed parts of Bangkok.
Two teenagers were cleared of involvement in the arson attack by a juvenile court in December due to a lack of witnesses.
About 90 people were killed and nearly 1,900 were wounded in a series of street clashes between demonstrators and security forces during the rallies.
The trial of 24 Red Shirt leaders on terrorism charges began in December but five of them enjoy immunity as they are now lawmakers, so the case is expected to take years to complete during breaks in the parliamentary session.
Abhisit Vejjajiva, who was prime minister at the time, was charged with murder in December along with his then-deputy Suthep Thaugsuban over the death of a taxi driver shot by soldiers during the violence.
The Red Shirts were demanding immediate elections, accusing Abhisit's government of coming to power unfairly in 2008 through a parliamentary vote after a court stripped Thaksin's allies of power.
Thaksin was ousted by royalist generals in a coup in 2006. He now lives in Dubai to avoid a jail sentence for corruption that he contends is politically motivated. In 2011 his sister Yingluck Shinawatra was elected as premier.