"We have decided to boycott the elections due to the electoral law and its unjust division of constituencies and the intervention of the great powers," the group said in a statement.
The complex electoral law guarantees parliamentary parity between the country's Christians and Muslims, even though the Christians are in a minority.
"Holding the elections on the basis of the prevailing law, which is criticised by the majority of political forces, will not allow fair elections ... and public opinion is shocked by the level of intervention by the great powers and the submission of the political class to its diktats," it added.
Jammaa Islamiya, traditionally seen as close to the assassinated Sunni former prime minister Rafiq al-Hariri, is an electoral force in majority Sunni cities such as Saida, Beirut and Tripoli.
Christian figures, including hardline leader Michel Aoun and Lebanon's Maronite bishops, have warned that the law will disrupt the country's fragile Christian-Muslim co-existence.
Meanwhile, former prime minister Omar Karameh, who tried to bring together a government in the aftermath of the al-Hariri assassination, announced he would not stand for re-election in his home city of Tripoli.
"Holding the elections on the basis of the prevailing law ... will not allow fair elections"
Jammaa Islamiya statement
Speaking in an interview with a private Lebanese television station, the pro-Syrian Karameh said he, too, did not agree with the electoral law and accused the opposition of using its financial superiority to unfair advantage during the campaign.
Karameh, who gave up the struggle to form a government on 13 April, said his allies were facing a severe challenge against an emboldened opposition that has managed to grab half the seats in Beirut due to a lack of opponents.
Lebanese elections are to be held on four consecutive Sundays from 29 May.