The man, a social services worker in Catanzaro in southern Italy, was convicted in February 2011 and sentenced to five years in prison for sexual acts with a minor.
The verdict was later upheld by an appeals court.
But Italy's supreme court ruled that the verdict did not sufficiently consider "the 'consensus', the existence of an amorous relationship, the absence of physical force, the girl's feelings of love".
The court's October 15th decision to order a retrial was made public this month by Il Quotidiano della Calabria and slowly spread to social media networks, where it sparked heated reactions against the Italian justice system.
According to Il Quotidiano, the girl came from a poor family who had known and trusted the social worker.
The man was caught naked in bed with the girl after an investigation by police based largely on wire-tap evidence, it said.