Speaking at St Peter's Basilica in the Vatican on the Catholic world day of peace he said everyone has a God given right to be free.
During his homily he said: "The message for the Day of Peace this year is 'No Longer Slaves, but Brothers and Sisters'.
"All of us are called to be free, all are called to be sons and daughters, and each, according to his or her own responsibilities, is called to combat modern forms of enslavement.
"From every people, culture and religion, let us join our forces."
The pontiff's comments come just weeks after he, along with other faith leaders, signed a declaration to work to end human trafficking.
The Global Freedom Network, which launched with backing of major faith leaders including Pope Francis and the Archbishop of Canterbury in March, is hoping to bring about a commitment to eradicate modern slavery by 2020.
As part of the campaign a 'Joint Declaration of Religious Leaders against Modern Slavery' was signed.
In an address before the signing, Archbishop Justin described the declaration as "a profoundly significant moment".
The leaders had gathered to "affirm a deep shared commitment for the liberation of those humiliated, abused and enslaved by their fellow-human beings," he said.
"There are already close and trusting relationships between us as faith leaders. Our task now is to make these relationships work effectively for the well-being of all people."