White silk cushions on which couples tie their wedding rings were handed out as gifts - complete with the pope's signature in Latin, "Franciscus" - during an unprecedented event entitled "The Joy of Yes Forever".
The Vatican said there were around 20,000 future brides and grooms attending from 25 countries, all of them enrolled on Catholic marriage preparation courses.
On a sunny day in Rome, couples were regaled with love songs and dance numbers from the stage in front of St Peter's Basilica and even a stand-up comic cracking jokes about chat-up lines before the pope's arrival.
Couples also told stories about how they first met, including two lovebirds who said they first exchanged glances while admiring the traditional Nativity scene set up on St Peter's Square before Christmas.
"This pope is really close to the way we live, to modern life, the new family, who we are," said Emmanuelle, a 29-year-old from Nantes in France on a Valentine's holiday with her atheist boyfriend Benoit.
Michael, 24, said he had left the Catholic Church because of the scandal of child sex abuse by priests but his fiancee Rosa, 21, was still a believer and said she thought the new Pope was "a great, positive change".
Valentine's Day is a secular celebration even though it celebrates a third century bishop, Saint Valentine, who is believed to have secretly married couples with a Christian rite despite persecution by the Romans. February 14th was the date of his martyrdom in the year 273.
"Today a lot of people are scared of making definitive choices," the Argentine pope said, adding: "Today everything is changing quickly, nothing lasts long".
"Living together is an art, a patient, beautiful and fascinating journey. It does not end when you conquer each other. In fact it is only just beginning," he said.
"Don't build your house on the sand of sentiments that come and go, but on the rock of real love," he added.
The Vatican invitation to the event organised by the Pontifical Council for the Family was decorated with silhouettes of a man and a woman embracing and a white cross with pink borders against a blue sky backdrop.
The celebration was not open to same-sex couples as Francis is opposed to gay marriage, even though he has called for a more tolerant approach to homosexuals.
During his time in office, Francis has put a strong emphasis on the traditional family and is holding two synods of world bishops on the theme in 2014 and 2015.
Lucy, 27, and Jan, 38, from the Czech Republic said they were already married but were attending the Vatican event because they believed the pope's prayer "could help" them in their longed-for wish to have a baby.
Another couple, Maria Flavia, 41, and Emanuele, 32, said they had been living together for two years but were not sure whether they would get married.
"We hope Pope Francis gives us a stimulus," she said. "Marriage is hard now. It's easy to get married and then separate and that's not what we want."