A statement released by the family of seven year-old Julian Cadman says the boy will be remembered as an "energetic, funny and cheeky" boy.
Julian Cadman became separated from his badly injured mother, Jom, in Thursday's van attack on the busy Las Ramblas boulevard.
She remains in hospital as one of the more than 100 people injured in the incident.
Another woman died in a separate attack at a nearby resort, while five attackers were fatally shot by police.
The news of the boy's death comes as Spanish police put up roadblocks across the country's northeast in hopes of capturing a fugitive suspected of being a member of the cell responsible for the two vehicle attacks.
Police say they believe the group had planned to fill the vans with explosives and create a massive attack in the Catalan capital, confirming more than 100 tanks of butane gas were found at a house in Alcanar, which exploded.
Catalonia police chief, Josep Lluis Trapero, says he can't rule out the possibility the suspect may have crossed into France.
"We are helping state security services responsible for controlling the borders - the Guardia Civil and national police - monitor the movements of people, and make the necessary checks. This clearly doesn't exclude the possibility that someone, perhaps using a vehicle - and I'm not saying we have any specific information but clearly it's a possibility - could have crossed a border by some means."
Spain's King Felipe and Queen Letizia have attended a mass held in honour of the victims of the attacks at the Sagrada Familia basilica.
Spain's Prime Minister Mariano Rajoy and regional leaders also attended the service, celebrated by Barcelona's Archbishop Juan Jose Omella, who shared a message of solidarity that Pope Francis had personally left on his cell phone.
"The Pope Francis wants to express his sadness for the victims that have lost lives in actions so inhumane. His offers his desire for eternal peace. In this moment of sadness and pain he offers his support and closeness to the wounded, to families, and all of Catalonia and Spain."
Police are hunting a Moroccan man suspected of driving the van: 22 year-old suspect, Younes Abouyaaqoub.
Police say an imam, Abdelbaki Es Satty, is also among the suspects.
He is believed to have radicalised youths in Ripoll, a small town at the foot of the Pyrenees, where four suspects - including Younes Abouyaaqoub - grew up or lived.
This Muslim resident of Ripoll says the community saw nothing to indicate the men would commit such an attack.
"They were just normal guys with normal jobs and lives. We had not seen them out much lately. But no-one suspected anything about them, no-one can believe what's happened, everyone believes someone must have brainwashed them."
Muslim community leaders have expressed their shock and disbelief over the incidents.
The head of Ripoll's Islamic association, Ali Yassine, says he never saw any signs of anything like this happening.
"He was a normal guy, he spoke, prayed normally; he assisted in Friday mass normally. Everything about him was normal, not a single unusual movement, nothing weird, not a single radical message, all normal."
It's the deadliest terror attack in Spain in over a decade.
The IS group has claimed responsibility for the attacks, believed to be its first in Spain.
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