The women had been rescued from their pimps and are being given shelter and protection at an apartment run by a Catholic charity in Italy's capital.
The pontiff chatted with the women, some trafficked from Africa including Nigeria and elsewhere in Europe, for more than an hour.
The 79-year-old cleric has repeatedly described human trafficking as a "crime against humanity".
The Pope sat down with the women, including seven Nigerians, six Romanians and four Albanians, and listened to their stories of forced prostitution. The other three in the group came from Italy, Tunisia and the Ukraine.
They were all aged about 30 and had "suffered serious physical abuse" and now lived under protection, the Vatican said.