Francis passed under the gates alone, wearing white robes and skullcap. After meeting the survivors, he placed a candle at the Death Wall, where prisoners were executed by the Nazis, before continuing on his own.
The Pope stopped to pray at the prison cell of Maximilian Kolbe, a Polish Catholic friar who sacrificed his life to save that of another man. The Church made him a saint in 1982.
Pope Francis knelt for many minutes in the underground cell, illuminated only by the light from a tiny window, the Associated Press news agency reports.
The Argentine Pope is on a five-day trip to Poland.
During a World Youth Day rally in the southern city of Krakow on Thursday, he urged compassion for migrants.
He told hundreds of thousands of people that “a merciful heart opens up to welcome refugees and migrants” – a statement that puts him at odds with Poland’s anti-immigrant right-wing government.