The Italian foreign ministry made the announcement as protesters blocked the road outside Alder Hey Children’s Hospital in Liverpool and tried to storm the entrance after European judges refused to intervene.
A statement added: “The Italian government hopes that being an Italian citizen will allow the child immediate transfer to Italy.”
Alfie’s father, who is said to be “heartbroken” following the ruling, spoke to supporters briefly outside the hospital to confirm the news of Italian citizenship and said it would block any planned withdrawal of life support.
Tom Evans said: “I’m stood here now and Alfie is still here. Why? Because I’m still fighting for him, I’m still fighting and so is Alfie.
“I have been in touch with the ambassador of Italy. My son belongs to Italy. I love Alfie and I love Kate, I will not give up.”
About 200 people gathered outside the building on Monday, chanting “save Alfie Evans”.
As dozens ran towards the hospital doors, police fanned out to block their entry.
After a tense stand-off, the demonstrators retreated to join others further away.
From the scene, Sky News Correspondent Gerard Tubb said: “Many of the protesters crowding the pavement outside the hospital, predominantly women, many with small children, are convinced that the authorities – from hospital medics to the Supreme Court – are part of a conspiracy to cover up medical malpractice.
“Chants of ‘Free Alfie’ and ‘murderers’ are interspersed with the blaring horns of passing cars and vans – their drivers expressing support for the protesters’ stand.”
The hospital said it remained “open as usual”, but added that “visitors may notice an increase in visible police presence”.
Chief Inspector Chris Gibson asked protesters to “respect families and staff, including the poorly children in the wards and to ensure that access to the hospital is not restricted at any time”.
“It should not be forgotten that many families are going through extremely challenging and emotional times,” he added.
Alfie, who was born in May 2016, is in a “semi-vegetative state”. His neurological condition has not been definitively diagnosed.
Specialists say his brain has been “eroded”.
The European Court of Human Rights said it had rejected an application to take the terminally ill toddler to Italy for treatment, describing it as “inadmissible”.
Last week, Mr Evans and Alfie’s mother, Kate James, lost a second appeal request for the Supreme Court to hear their case.
In February, Mr Justice Hayden ruled that doctors could stop treating Alfie against the wishes of his parents.
But Mr Evans and Ms James say Italian doctors are willing to treat him and an air ambulance is available.