The Russian-brokered deal was reached on Wednesday after a meeting between a Syrian government delegation and representatives of residents and fighters in rebel-held Harasta, Al Jazeera reported.
"Evacuation for families who want to leave will begin tomorrow at 7 am (05:00 GMT)," Munther Fares, spokesperson for the Ahrar al-Sham group, which holds Harasta, said in a statement on Wednesday.
"Gunmen and civilians who choose to leave will have Russian guarantees."
Addounia TV, a state-run outlet, said that 1,500 gunmen and 6,000 family members would be evacuated on Thursday in two groups to Idlib.
Harasta is home to 20,000 people.
The evacuation deal marks the first time opposition fighters have been able to leave rebel-held Eastern Ghouta, which has been subjected to an aerial offensive carried out by Syrian government forces and Russian fighter jets for more than a month.
Tens of thousands of civilians have fled Eastern Ghouta, which used to have a population of around 400,000.
Eastern Ghouta fell to militants in 2012, months after Syria plunged into crisis, and has since served as a launch pad for fatal mortar attacks against residents and infrastructure in the capital.
The army further called on civilians in Eastern Ghouta to leave the remaining militant-held districts through the safe corridors provided by the government forces.