In a report published by the science journal, Eurosurveillance, on Thursday, the doctors said the virus was brought to Florence in May 2014 by an Italian man, who, after returning from a holiday in Thailand, developed a fever and rash.
The symptoms subsided within a week, but almost three weeks later his girlfriend, in her late 20s, developed similar symptoms.
The pair had sexual intercourse in between the time the man's rash and fever had subsided, and the onset of his girlfriend's symptoms. At the time, doctors diagnosed both with Dengue fever, which they said comes with similar symptoms to Zika.
But as cases of the rapidly spreading Zika, a tropical virus that is usually transmitted by the bite of an infected mosquito known as Aedes Aegypti, started to be reported worldwide last year, the medics twice reanalyzed serum samples from both patients.
They said in the report that the most recent analysis, carried out this month, “showed positive signs for ZIKV neutralising bodies...with a clear increase in the antibody titre between the first and second serum sample for both patients”.
The doctors claim their findings provide “further evidence that sexual transmission of Zika virus is possible” even though “little evidence supports the possibility of Zika sexual transmission to date”.
Their report came during the same week US health authorities said they were investigating 14 new cases of Zika, which had possibly been sexually transmitted.
Scientists have, to date, maintained that sexually transmitted cases are extremely rare.
Cases of the virus, which is thought to deform fetuses, have also almost exclusively originated from South America, namely Brazil, Colombia and El Salvador.
Four cases were recorded in Italy in January, with the virus diagnosed in people who had recently returned from South America.