"The Eritreans for years made the cross here [indicating the forehead]. Also today we see them. The cross was tattooed. Yes, they are exaggerations, but today I see some," he said.
He added that young priests could use them to connect with the people they were trying to reach out to, because the tattoos often communicated information about the person.
A tattoo can become a talking point and a way for the priest to find out more about an individual, he said, and to connect with "the culture of the young".
He said: "Tattoos often signify membership in a community. 'You, young man, that you're tattooed like that, what are you looking for? In this tattoo, which community membership are you expressing?'
"It's important not to be scared. With young people one should never be scared! Never! Because always, even behind the things that are not so good, there is something that will bring us to some truth."
Christians are divided on whether tattoos are acceptable, and there is no clear-cut teaching on the topic.
The Bible only includes one possible mention of tattoos, in the Old Testament book of Leviticus, which includes the command: "Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you".
Some Christians have interpreted this as forbidding the use of modern tattoos, while others argue that the verse is not binding upon believers because it was only relevant to the cultural context of the day, and appears alongside other commands which are no longer followed, including for men not to shave the edges of their beards or cut the hair on the sides of their heads.
The New Testament does not contain any explicit mention of tattoos, though some have linked them with the Mark of the Beast mentioned in the book of Revelation.
The Pope's comments, which were originally reported by the website ChurchPop, were made ahead of the Synod, which will be focused around the topic Young People, Faith and the Discernment of Vocation.
In response to questions from other attendees the Pope condemned Catholics who pay for prostitutes and said the practice was a "crime against humanity".
He asked forgiveness for all Catholics who participated in these "criminal acts".
In his opening address to the group, he said young people "must be taken seriously" and are often "marginalized from ordinary public life".
"It seems we are surrounded by a culture that, if on the one hand it idolizes youth, trying to prevent it from passing by, on the other it excludes many young people from being active agents," he said.
