Ribat the Archbishop of the Port Moresby diocese, called the decision "responsible" and the "right thing to do", The National newspaper reported.
He was responding to Prime Minister James Marape's remarks that being a Christian nation, the Bible commanded that "thou shalt not kill".
All those on death row now will be given life sentences without parole.
Robat told The National that the Catholic church was happy to hear the news.
"My view is that we have the right to respect, promote and protect life, and on the human rights of people, we have to ensure we do the right thing by ensuring those in prison are given a second chance," he said.
"The way to ensure these prisoners are rehabilitated is to ensure that capital punishment is (commuted) to life sentence. This will allow prisoner to appeal for mercy.
"When they appeal for mercy, and according to assessment by the prison authorities and the parole board, they can fit back into the community as long as they have improved."
He said prison officers should be well trained to assist prisoners on death row to find a purpose in life.
"When they are trained to help such prisoners, we can expect changes in their lives," he said.