The National Coordinating Committee for Democratic Change (NCCDC), which is tolerated by the regime in Damascus, said it had "decided to boycott" the April 13 parliamentary polls.
Group members will neither run for office nor cast their ballots, the group said in a statement posted on Facebook, while calling on other "opposition forces and civil society to join the boycott".
The NCCDC accused the government of seeking to "improve the conditions" of UN-mediated indirect talks in Geneva between the regime and opposition.
The latest round of negotiations aimed at ending Syria's war are to begin on Monday, according to UN peace envoy Staffan de Mistura.
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The focus will be the planned formation of a new government, the drafting of a new constitution and the organisation of fresh presidential and parliamentary elections within 18 months.
Syria last held a parliamentary vote in May 2012.
That was the first time that multiple parties -- not just President Bashar al-Assad's ruling Baath -- were allowed to stand.
Most of the 250 members of parliament elected for four-year terms were Baath members.
At the time, Assad appointed then-agriculture minister Riad Hijab to be Syria's new prime minister.
Hijab has since defected and now leads the Riyadh-based High Negotiations Committee, a broad opposition grouping that includes the NCCDC.