"Dear young friends, we have an appointment for the next World Youth Day in 2016 in Krakow, Poland," he said at the end of a mass attended by three million pilgrims on Rio's Copacabana beach.
Argentine-born Francis was wrapping up a week-long visit to Brazil, the world's most populous Catholic country, on his first overseas trip since his election in March.
The choice of Krakow is in honor of John Paul II, who hailed from Krakow and who started the World Youth Day (WYD) events during his charismatic papacy.
John Paul II, who is expected to be declared a saint later this year, was archbishop of Krakow before he was elected the world's first Polish pope in 1978.
The first official WYD was held in 1986, although it had been preceded by two previous mass gatherings of young people from around the world in Rome.
The next was held the following year in the Argentine capital Buenos Aires, where Francis was archbishop before he became pope.
Latin America's first pontiff was given a rock star welcome on Rio de Janeiro's Copacabana beach, with the faithful waving flags, dancing and chanting "long live the pope!" in a Carnival-like atmosphere.
The head of the world's 1.2 billion Roman Catholics hammered home his plea for young believers to "go and make disciples of all nations" during a trip aimed at stirring passion in a faith losing followers in nations like Brazil.
"Do not be afraid to go and to bring Christ into every area of life, to the fringes of society, even to those who seem farthest away, most indifferent," the 76-year-old Argentine said as dozens of people took a dip in the ocean.
The mass included usual Bible readings but also a rousing concert with a band and choir that could have been seen in one of Brazil's many and expanding Evangelical churches that have lured Catholics.
The crowd estimate provided by organizers of World Youth Day, a gathering of young Catholics, was more than twice the number of people who attended a free Rolling Stones concert in 2006.
"World Youth Day was fantastic. Everybody is united in God's path. Now the pope leaves and we have to evangelize," said Andressa Pusak, 25, of Brazil.
Francis, who flies back to Rome in the evening, arrived by helicopter and then cruised by the crowd in an open-sided popemobile that took him to the giant stage.
Voice of Russia, AFP