Pope Francis further admitted the mistreatment of women was a "cultural problem" and allegations about nuns being abused were being individually investigated.
The Pope's comments, which came during a news conference aboard the papal plane on a return flight to Rome from the United Arab Emirates, come as the Catholic Church is dealing with sexual abuse scandals on several continents.
Pope Francis, who canonised Paul VI in October 2018, approved the optional memorial in light of "the petitions and desires of the people of God", said the decree, signed by Cardinal Robert Sarah and Archbishop Arthur Roche, the congregation's prefect and secretary, respectively.
The abuse of adult nuns-like the child abuse scandals rocking the church-has also been a global one. It's not a thing that from the moment in which you realize it, it's over.
But many Catholics say abuse of vulnerable adults, including nuns and seminarians, has always been a problem in the church.
File image of Pope Francis.
Last year, the French website La Parisien reported the case of a former nun who was abused and raped by her parish priest between 2010 and 2011.
NPR: "Pope Francis Acknowledges, For First Time, Sexual Abuse Of Nuns By Priests" - "Pope Francis, for the first time, acknowledged the sexual abuse of nuns by priests and bishops, including a case in which some clergy used women as sex slaves". Some hope the church will address the issue at the upcoming meeting on the abuse crisis to be convened by Pope Francis from February 21 to 24 in Rome.
In November, the International Union of Superiors General - the organization representing all the world's female Catholic religious orders - publicly denounced the "culture of silence and secrecy" that prevented nuns from speaking out and urged sisters to report abuse to their superiors and police.
Francis said Benedict acted "because a certain slavery of women had crept in, slavery to the point of sexual slavery on the part of clergy or the founder".
Last year, the Associated Press discovered cases of nuns having been sexually abused in Europe, Africa, South America and Asia.
"I think it's still going on, because it's not something that just goes away like that".
Vatican spokesman Alessandro Gisotti said Francis "spoke of "sexual slavery" to mean "manipulation" or a type of abuse of power that is reflected in a sexual abuse".
Francis said the sisters had been reduced to "sexual slavery" at the hands of the Reverend Marie-Dominique Philippe and other priests.
"I want to move forward".
In a separate case in India a year ago, a bishop was arrested over allegations that he raped a nun 13 times between 2014 and 2016.
She added: "He was unable to control himself... he had a split personality".