Pope Francis spent time with about 90 inmates at the Giuseppe Salvia Detention Center in Poggiorale, Italy on Saturday.
The inmates, who were chosen at random, included 10 men from the prison's ward for gay, transgender, and HIV-positive convicts.
The Religion News Service reported that Francis insisted on having lunch with the prisoners, although it was not originally on his Naples itinerary.
Reporters were not allowed inside the luncheon, but the Italian media reported that the inmates prepared pasta. Prison ministry leader Anna Maria Esposito told Vatican Radio that Francis wanted to spend time with the prisoners individually, and insisted upon a "frugal" lunch. The meetings were held in a room that is used as the prison's chapel.
During a sermon at St Peter's Basilica last month, the pontiff warned against having a "prejudiced mentality" that marginalises others, and used the biblical story of Jesus healing a leper as an example of how Christians should treat society's outcast.
"Jesus responds immediately to the leper's plea, without waiting to study the situation and all its possible consequences," Francis told hundreds of cardinals and bishops.
"For Jesus, what matters above all is reaching out to save those far off, healing the wounds of the sick, restoring everyone to God's family."
He has also urged the Church to support families of gays, and famously quipped "Who am I to judge?" when asked about homosexuality in a 2013 press conference.
Francis' trip to Naples also included a visit to Scampia, which is largely run by members of the Camorra crime family. The Pope called for more opportunities for residents to earn a fair wage in legal employment, and denounced the exploitation of workers.
Pope Francis has consistently called for unity against injustice, and has made the plight of underprivileged people and victims of modern slavery priorities for the Vatican.
