In a speech to the International Committee against the Death Penalty the leader of the world's Catholics said: "Nowadays the death penalty is inadmissible, no matter how serious the crime committed."
The Pontiff added that state execution represents a failure of the law because it makes governments 'kill in the name of justice'.
He said his thoughts had come from the Church's teaching on the importance of human life.
Pope Francis accepted people had to be punished for their crimes but argued: "When the death penalty is applied, it is not for a current act of aggression, but rather for an act committed in the past.
"It is also applied to persons whose current ability to cause harm is not current, as it has been neutralized, they are already deprived of their liberty."
The Holy Father was also critical of whole life sentences: "Life imprisonment entails for the prisoner the impossibility of planning a future of freedom, and may therefore be considered as a sort of covert death penalty."
Previously Pope John Paul II had said the death penalty was acceptable in some extreme cases 'of absolute necessity'.
"In other words, when it would not be possible otherwise to defend society," John Paul added.
The death penalty has been used in almost every country of the world at some point but is now outlawed in many places including the UK.
Execution is still legal in some US states.