Maria Giulia Sergio, a 27-year-old who now goes by the name of Fatima Az Zahra, is thought to have left Italy for Syria last September with her Albanian husband, Corriere reported.
Her husband reportedly had ties to Bilal Bosnic, the Salafi movement leader in Bosnia Herzegovina who was last year arrested for funding terrorist activities as well as recruiting and fighting for Isis, the newspaper said.
Sergio is the only woman among the four Italian citizens to have joined the extremists.
Another was Genoese Delnevo, a 23-year-old from Genoa who died in Syria in June 2013.
A further 49 people have transited through Italy on the way to joining Islamic extremists, Italy's Interior Minister Angelino Alfano said last week.
Born in Torre del Greco, near Naples, Sergio later moved with her family to Inzago, a town near Milan.
She converted to Islam after her first marriage to a Moroccan citizen. After meeting her second husband, she moved to Grosseto, in Tuscany, where at the end of 2012 she underwent the rituals of Islamic radicalization, Il Tempo reported.
There she “changed her name to Fatima Az Zahra, visited mosques and wore a niqab”, the website said.
Sergio also reportedly used her Facebook page to defend Islamic traditions and ask for "victory" over "infidels".
Her parents and sister, who still live in Inzago, also converted to Islam, Il Fatto Quotidiano reported.
Her mother, Assunta Bonfiglio, was on Tuesday quoted by La Repubblica as saying that she hasn't had any contact with her daughter and doesn't know where she is but that “Allah is protecting her”.
“She is not a terrorist hiding behind a veil,” she added.
“My daughter Fatima is good, and those who know her can attest to that.”
The couple are thought to have flown from Rome to Istanbul in September, before making their way over the Turkish border into Syria.
Hundreds of women have joined the ranks of Isis.
About 60 British women are thought to have joined, while others are known to have travelled from Sweden, France, Belgium, Canada and the US.
In December, Spanish police arrested four women and three men in Spain and Morocco who were part of a network recruiting women to send over to Isis.
In the wake of the terrorist attacks in Paris last week, Alfano said fresh legislation would target "foreign fighters" seeking to travel from or through Italy en route to joining Islamic militants.
The intention is to "restrict their ability to go into the field", he said, with the new law enabling the Italian government to take away suspects' passports.