The damage to an entire generation of children could soon become irreversible without immediate help, it adds.
The stress of war has led to increased bedwetting, self-harm, suicide attempts and aggressive behaviour among many children, according to a new report.
The findings are based on hundreds of interviews in Syria.
Save the Children says its study is the largest of its kind into the mental health and well-being of Syria's children amid the war, which began in 2011 and has left more than 300,000 people dead.