"But how can an act that suppresses innocent and defenseless budding human life be therapeutic, civil or simply human?" the pontiff said.
He asked if it was right "to do away with a human life to solve a problem," adding: "Is it right to hire a hitman to solve a problem? One cannot do this, it is not right to do away with a human being, albeit small, to solve a problem."
It's not the first time the Pope has spoken out on abortion, a practice that goes against the traditional teachings of the Roman Catholic Church.
"I have heard that it's fashionable, or at least usual, that when in the first months of pregnancy they do studies to see if the child is healthy or has something, the first offer is: let's send it away," Francis was reported as saying in remarks to Italy's Family Association in Rome.
"I say this with pain. In the last century the whole world was scandalized about what the Nazis did to purify the race. Today we do the same, but now with white gloves."
The Nazi quest for a pure race by allowing only healthy children to be born, known as eugenics, included abortion and the sterilization of people with physical and mental illness.
CNN's Hada Messia reported from Rome and Laura Smith-Spark wrote from London.