Polish media broadcast images of the 79-year-old Argentine pontiff stumbling, then falling down out of view before being helped to his feet by altar servers.
Francis appeared to be fine as he delivered a homily and said mass for hundreds of thousands of pilgrims.
The mishap took place as the pontiff visited the revered Jasna Gora monastery in Czestochowa, southern Poland.
It is home to the legendary Black Madonna, an icon which Catholic faithful believe has miraculous qualities.