Moaz al-Khatib, ex-leader of the main opposition Syrian National Coalition, who recently established his own dissident movement, had visited Russia last year for discussions.
But on Friday, he said via his Facebook page that "the necessary conditions" for a dialogue have not yet been met.
Khatib also cited the regime's ongoing shelling of opposition-held areas in Syria as a reason for his refusal to attend the Moscow talks slated for later this month.
"I received a written invitation from the Russian leadership to meet with several Syrian personalities in Moscow this month, and after consultation with the gathering I work with, I decided to decline," said Khatib.
"The conditions we believe are necessary for success have not been met, neither has the call for the shelling and killing of our people been heard," he said.
Khatib added: "While we do not reject the prospect of any future meeting with a regime delegation, we would require the regime to release the detainees, especially the women and children."
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Khatib's statement comes four days after the newly elected head of the National Coalition, Khaled Khoja, said the main opposition grouping had ruled out taking part in the talks.
Russia, a key ally of Syrian President Bashar al-Assad, has been trying to relaunch peace talks that would include meetings between delegates of the regime and the fractured opposition.
It invited 28 opposition figures, including members of the tolerated domestic opposition as well as individual Coalition members, to Moscow in January.
It is unclear how the talks will go ahead now that the Coalition and Khatib have both ruled out attending.
Syria's war began as a peaceful, pro-democracy movement, but later morphed into a brutal civil war after the regime unleashed a massive crackdown against dissent.
Two rounds of talks in Geneva, Switzerland last year brought together the regime and the opposition, but failed to secure any results.
The war has killed more than 200,000 people and forced half the population to flee their homes.