About 1.5 million people attended his closing Mass at a riverside park in the Portuguese capital, the Vatican said, quoting local authorities.
Many of the faithful slept outdoors, having attended a vigil there on Saturday night, and gathered in sweltering heat.
Speaking after the Mass, the 86-year-old Francis urged the young people to take the fraternal experiences of the six-day jamboree back home and apply them to their daily lives.
"Dear friends, allow me, this old man, to share with you young people a dream that I carry within me - it is the dream of peace, the dream of young people praying for peace, living in peace and building a peaceful future," Francis said.
"As you return home, please continue to pray for peace.
"What is more, you are a sign of peace for the world, showing how different nationalities, languages and histories can unite instead of divide.
"You are the hope of a different world," he said.
The Pope asked them to think of the young people who could not come to the event because of the world's many armed conflicts and wars, adding: "In thinking of this continent, I feel great sorrow for beloved Ukraine, which continues to suffer greatly."
Francis, who was returning to Rome on Sunday evening after an event to thank volunteers at the World Youth Day festival, met a delegation of 15 young people from Ukraine during his trip.
Sunday's Mass was concelebrated by 700 bishops and 10,000 priests, who distributed communion to the huge crowd.
The Pope announced the next World Youth Day would be held in Seoul, South Korea in 2027.
One of the recurring themes of the Pope's visit was social media and its potentially negative effects on young people.
During the week, Francis urged them to beware of the false happiness lurking in the virtual world, and at another event, the young people themselves reflected on their anxieties, enslavement to the "tyranny" of social media and yearning to save the planet.
The trip took place in the shadow of a report six months ago by a Portuguese commission that said at least 4815 minors were sexually abused by clergy - mostly priests - across seven decades in the country.
It was just one in a series of reports around the world that have exposed clerical sex abuse and rattled the Catholic Church in recent years.
Francis said on Wednesday the church needs a "humble and ongoing purification" to deal with the "anguished cries" of victims of clerical sexual abuse, and met privately with 13 victims.
