The pope listened intently as a Polish priest who accompanied Lidia Maksymowicz, 80, told him of her story.
She then rolled up her left sleeve to show him the number - 70072. He kissed it and she hugged him.
Maksymowicz and her family were taken from their home in Belarus to the Nazi death camp in German-occupied Poland in December 1943, shortly before her third birthday.
She was put in a children's barracks, where she and others were the subjects of medical experimentation by Doctor Josef Mengele, according to a documentary about her life.
After the liberation of the camp in 1945, Russian soldiers assumed her mother Anna - tattooed with the number 70071 - was dead. She was adopted and raised by a Catholic Polish family.
Born Ludmila Boczarowa, she did not know her birth mother had survived and they were briefly reunited shortly before her mother's death in the early 1960s.
Maksymowicz, who lives in Krakow, Poland, is the subject of a documentary called "70072: The Girl Who Couldn't Hate. The true story of Lidia Maksymowicz."
She often meets young people in schools to discuss the dangers of extremism and populism.
The Nazis and their allies murdered around 6 million Jews, as well as others, in German-occupied Europe.
More than a million people, most of them Jews, were killed at Auschwitz. The vast majority were gassed to death.
Francis, who was holding a general audience in the Vatican's San Damaso Courtyard, visited Auschwitz in 2016.