There had been some confusion over his fate after his grandfather and cousin issued a Facebook appeal hours after the attack on Thursday, claiming he was missing.
But police in Catalonia later insisted that the boy had been on their list of 13 fatalities but they were awaiting for formal identification before making the death public.
3 more fatalities of the atttack in #Barcelona identified: 1 Australian boy (also UK nationality) 1 Italian citizen 1 Belgian— EmergènciesCatalunya (@emergenciescat) August 20, 2017
Julian’s father arrived in Barcelona on Saturday afternoon after a flight from Sydney and was met by Australian consular officals before being taken directly to the morgue at the Ciutat de Justicia.
He was then taken, with police car escort, to the hospital where his wife and the boy's mother, Jumarie "Jom" Cadman is being treated for injuries sustained in the terror attack on Barcelona's Las Ramblas boulevard, where a van driving at high speed mowed down more than 100 people, and killed 13.
In a heart-wrenching statement, Julian's family described him as "a much loved and adored member of our family"
"As he was enjoying the sights of Barcelona with his mother, Julian was sadly taken from us.
"He was so energetic, funny and cheeky, always bringing a smile to our faces. We are so blessed to have had him in our lives and will remember his smiles and hold his memory dear to our hearts," they said, thanking those who helped in the search for the boy.
"We will not be making any further comments on this tragedy and ask everyone, especially the media, to give us the privacy we need to grieve away from the public eye."
Explaining the delay in announcing the death, a Catalonian health department source said the child's identity could not be confirmed until his father arrived from Australia and was informed about the death.
Sunday's confirmation by emergency services brings the total number of victims identified to 12.
Besides the Australian-British child, they include four Spaniards, two Portuguese, two Italians, an American, a Spanish-Argentinian and a Belgian.
Among the 126 people injured in the twin attacks, 51 are still hospitalised, including 10 in a critical condition and 13 in a serious state.