Myanmar reporters Wa Lone, 31, and Kyaw Soe Oo, 27, were arrested last December and have been charged under the Official Secrets Act, which could see them jailed up to 14 years.
They had been investigating the killing by security forces and ethnic Rakhine locals of 10 unarmed Rohingya Muslim men in the country's crisis-hit northern Rakhine State.
"Wa Lone and Kyaw Soe Oo are being prosecuted simply because they reported the news. I have reviewed the case file and it is clear beyond doubt that the two journalists are innocent and should be released immediately," Clooney said.
"Yet they have been denied bail and face 14 years in prison. The outcome of this case will tell us a lot about Myanmar's commitment to the rule of law and freedom of speech."
Clooney's involvement will increase the global attention that has surrounded proceedings against the pair, who are being detained in a Yangon prison.
"We will pursue all avenues to secure our reporters' release. Retaining Ms Clooney greatly strengthens our international legal expertise and allows us to broaden those efforts," said Gail Gove, chief counsel of Reuters.
Former US president Bill Clinton and UN Secretary General Antonio Guterres are among those who have called for the journalists' release as fears grow about Myanmar's shrinking press freedoms.
Clooney, an internationally respected lawyer, has also attracted huge celebrity following since her 2014 marriage to American film star George Clooney.
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