The event was created by the late Pope John Paul for young Catholics in their teens or early 20s.
It is held every two or three years in a different city and is often dubbed the “Catholic Woodstock.”
This year’s event is the first time it has been held since 2019 due to the coronavirus pandemic.
The pontiff will take part in a welcoming ceremony at Figo Maduro Air Base in Lisbon before visiting the Portuguese president, Marcelo Rebelo de Sousa, later this morning.
As part of the festival, young people will meet for Masses, vigils and other social and religious gatherings.
Around 330,000 young people have registered and many more are expected to attend, the Vatican has said.
Portuguese authorities have predicted more than one million attendees.