The photo shows the 88-year-old pope, bare-headed without his customary white skullcap and wearing a white robe and purple stole. He is seated in a wheelchair in front of a simple altar with a crucifix on the wall.
The Vatican said it had been taken Sunday morning.
Taken from behind Francis' right side, his face is not fully visible but his eyes are open as he looks in a downward direction.
"This morning Pope Francis concelebrated the Holy Mass in the chapel of the apartment on the tenth floor of the Gemelli Polyclinic," the Vatican press office wrote in the photograph's caption.
Concelebration is the joint celebration of mass by senior clerics.
- 'Our bodies are weak' -
Since his hospitalisation, Francis has been unable to preside over mass at St Peter's Basilica in the Vatican. Nor has he personally delivered the Angelus prayer that follows the mass to the faithful gathered in St Peter's Square.
But earlier Sunday, in an Angelus message published by the Vatican, he thanked well-wishers while acknowledging his fragile health.
"I am sharing these thoughts with you while I am facing a period of trial, and I join with so many brothers and sisters who are sick: fragile, at this time, like me," wrote Francis.
"Our bodies are weak but, even like this, nothing can prevent us from loving, praying, giving ourselves, being for each other, in faith, shining signs of hope," he added.
The message marked the second Sunday of Lent, a 40-day period of prayer and reflection leading up to Easter.
Francis has improved steadily over the past week, the Vatican saying Saturday his condition continued to be stable, although he still required therapy to be administered from the hospital.
Although he has yet to appear at the window of his papal suite on the 10th floor of the Gemelli hospital, this has not dissuaded a steady stream of well-wishers from gathering, including tango dancers and dozens of children on Sunday.
About a dozen couples in street clothes danced the tango under grey skies in front of a throng of cameras near the hospital entrance.
"With this tango, he must be discharged," enthused dancer Daiana Guspero, 38, who like the pope hailed from Buenos Aires.
"I want him to feel our energy, our love for tango and for an Argentine pope," she told AFP.
- 'The pope of the children' -
Earlier, a group of young scouts from a Catholic group stood by a statue of former Pope John Paul II at the entrance, holding yellow and white balloons and vainly striving to catch a glimpse of the pope.
Group leader, Valerio Santobonio, 23 said the five to seven-year-olds did not quite yet grasp who the pope is, nor his health situation.
Nevertheless, he added, their visit was "a bit like giving them a window onto a wider stage of Christian life".
Other children had arrived in the early morning from an impoverished town near Naples to deliver a letter to Francis, said Andrea Lacomini from UNICEF, which organised the excursion.
"He loves children, he is the pope of the children, so we are waiting for him. We're sure he will get better," Lacomini told AFP.
"We need an important leader like him, because at this time there aren't many heroes in the world," he added.
"He's the only one who talks about peace."
Francis addressed his youngest well-wishers in his message.
"I know that many children are praying for me; some of them came here today to 'Gemelli' as a sign of closeness," he wrote.
"Thank you, dearest children! The pope loves you and is always waiting to meet you."
- 'Loving care' -
Last week the Vatican signalled that Francis was out of immediate danger after a series of breathing crises earlier in his hospitalisation had sparked fears for the Jesuit's life.
Although the Vatican has said that Francis continues to work from his hospital suite when able, his absence is particularly felt as Easter approaches.
Easter, the holiest period in the Christian calendar when the leader of the world's Catholics presides over a busy programme of events.
In his written message Sunday -- which also called for peace in war-torn countries including Ukraine, Myanmar and Sudan -- Francis again thanked those caring for and praying for him.
"How much light shines, in this sense, in hospitals and places of care! How much loving care illuminates the rooms, the corridors, the clinics, the places where the humblest services are performed!" he wrote.
And in a message sent by the Vatican's secretary of state, he offered his condolences to families of the victims and survivors of the deadly fire in a North Macedonian nightclub.
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© Agence France-Presse
