Pope says ex-pontiff Benedict 'very ill', asks for prayers
Benedict, who in 2013 became the first pontiff to resign in six centuries, has almost entirely withdrawn from public view.
The few photographs that have emerged have shown him to be in increasingly frail health.
"I would like to ask all of you to pray a special prayer for Pope Emeritus Benedict," Francis said at the end of his general audience.
He called on people to "remember him, because he is very ill, asking the Lord to console and support him".
Benedict had cited his declining physical and mental health back in 2013 in his decision to become the first pope since 1415 to give up the job as head of the worldwide Catholic church.
The German pope emeritus, whose real name is Joseph Ratzinger, has been living a quiet life in a former convent inside the Vatican.
His resignation created an unprecedented situation in which two popes -- Benedict and his successor, Pope Francis -- have co-existed within the walls of the tiny city state.
In April, Benedict's long-time secretary, Archbishop Georg Gaenswein, told Vatican News the ex-pope was "physically relatively weak and fragile", but "in good spirits".
God's Rottweiler
Benedict was 78 when he succeeded the long-reigning and popular John Paul II in April 2005.
His papacy was beset by Church infighting and outcry over paedophilia.
He became the first pontiff to apologise for the scandals that emerged around the world, expressing "deep remorse" and meeting with victims in person.
But while he took key steps to tackling clerical child abuse, he was criticised for failing to end Church cover-ups.
The abuse scandal has returned to haunt him in retirement.
A damning report for the German church in January 2022 accused him of personally failing to stop four predatory priests in the 1980s while archbishop of Munich.
Benedict has denied wrongdoing and the Vatican has strongly defended his record.
Unlike his successor Pope Francis, a Jesuit who delights in being among his flock, Benedict is considered a conservative intellectual.
He was dubbed "God's Rottweiler" in a previous post as chief doctrinal enforcer.
But as pontiff he appeared overwhelmed by the challenges facing a Church that was losing influence and followers, and years of Vatican turmoil took their toll.
He stepped down in February 2013 in an announcement delivered to cardinals in Latin, later saying that the decision was the result of a mystical experience.