The man, 24-year-old Anas al-Basha, was reportedly killed in an air strike.
Mr al-Basha was a centre director for civil society group, Space of Hope, and dressed up as a clown to bring comfort to children in eastern Aleppo.
Government forces have been staging air strikes on the eastern districts of the city, with troops entering the area over the weekend.
Dozens have been killed in the renewed fighting.
Mr Basha reportedly died on Tuesday in the Mashhad neighborhood.
In a post on Facebook, a man who identified himself as Anas' brother, Mahmoud al-Basha wrote "He lived to make children laugh and happy in the darkest most dangerous place in the world."
"Anas who refused to leave Aleppo and decided to stay there to continue his work as a volunteer to help the civilians and give gifts for the children in the streets to bring hope for them."
A post by the brother of Anas al-Basha, announcing his death (Facebook) Source: Facebook
Mr Basha had recently married, and his wife is still trapped in the city.
More than 25,000 people have fled Aleppo since fighting intensified in the rebel-held area.
The news of his death has sparked an outpouring of grief on social media.
One user tweeting, "This is so undescribable (sic) sad."
Tweet Source: Twitter
