The deaths come despite the area falling within a so-called "de-escalation zone" put in place under a deal between government allies Russia and Iran and rebel backer Turkey.
Eastern Ghouta is already in the grip of a humanitarian crisis caused by a crushing regime siege of the area since 2013 that has caused severe food and medical shortages.
Sunday's air strikes on the towns of Mesraba and Madira killed 21 civilians, while artillery fire on the town of Douma killed two others, the Britain-based Syrian Observatory for Human Rights monitor said.
Several people were also wounded in the air strikes, and an AFP reporter who visited a hospital in Mesraba saw doctors and nurses treating those injured.
Among them was a baby whose head was wrapped in a blood-stained bandage, as well as men and children who sat on the floor as they received first aid.
A small girl cried as a doctor bandaged her head while nearby a man sat against a wall, sobbing silently with his face pressed against his folded arms, the reporter said.
In a room of the hospital a morgue staff is placing identity tags on bodies wrapped in white sheets, victims of the air strikes, the reporter added.
"The toll could rise further because of the number of wounded people in a serious condition," said Observatory director Rami Abdel Rahman.
Among the dead were four children, the Observatory said.
Regime bombardment of Eastern Ghouta in the last two weeks has killed more than 100 people, according to the Observatory.
Rebels have also fired from the region into Damascus, killing several people.
Humanitarian access to Eastern Ghouta has remained limited despite the implementation of the truce zone, and a United Nations official referred to the region as the "epicentre of suffering" in Syria.
More than 340,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.