The monarch, 74, toured the venue,at the Ukrainian Catholic Cathedral of the Holy Family in Exile, with Ukraine’s first lady Olena Zelenska.
On his second visit to the cathedral since Russia's invasion of Ukraine in February, the king was given a traditional Ukrainian welcome of bread and salt before being invited to meet staff, volunteers and refugees.
Bishop Kenneth Nowakowski, head of the cathedral, told The National that the joint visit had given an “incredible morale boost” to those who fled the invasion and were establishing new lives in the UK.
He said Mrs Zelenska's appearance at the centre also showed refugees “they have not been forgotten” by those in their homeland.
“This was the second visit of his majesty to our cathedral since the full-scale invasion and it has shown us how much we mean to the royal family,” the bishop said.
He said Ukrainians had taken the opportunity to thank the king for “the openness and the kindness” shown by the people of Britain.
The Ukrainian Catholic Eparchy of the Holy Family in London and the Association of Ukrainians in Great Britain work together to offer assistance to refugees fleeing the war at the centre.
Vadym Prystaiko, Ukraine’s UK ambassador, was among those who met the king and President Volodymyr Zelenskyy's wife during the visit.
Bishop Nowakowski showed King Charles a religious icon painted on an ammunition box that had come to the cathedral from Ukraine.
The head of Britain's royal family was also shown how Ukrainians are benefiting from English language classes, art therapy and a mother and toddlers' club.
Khrystyna Kazarian, who volunteers at the welcome centre, said the king showed great interest in the work being done.
She told the PA news agency that her daughter tried to jump on the monarch when he approached her for a chat.
“He asked what we are doing now,” she said. “My daughter actually tried to jump on him several times.
“He said, 'I can’t imagine how you are doing this’.”
“He was very nice and warm,” she added.
Mrs Zelenska also toured the centre, speaking to attendees and volunteers.
After circling the room, King Charles was brought to the front to unveil a plaque commemorating the opening.
Bishop Nowakowski thanked him for his support.
“Your majesty, I have the great pleasure of welcoming you to our Ukrainian Welcome Centre,” he said.
“I also want to express my gratitude to you, sir, for being here right after the invasion of Ukraine in the first days of March, with her majesty the Queen Consort.
“This really inspired us, this really gave us a lot of assistance.”
He said he hoped that the next time the king visited would be after Ukraine had won the war.
King Charles unveiled the plaque, which read: “To commemorate the official opening of the Ukrainian Welcome Centre by His Majesty King Charles III.”
He then wished the attendees a Happy Christmas in Ukrainian.
Bishop Nowakowski also presented the king with a gift — a framed photograph of him dancing the traditional Ukrainian Hopak dance in 1981 with a group of folk dancers in Derby.
“We have a picture that has become famous worldwide,” the bishop told the king. “I have to say that throughout the Ukrainian diaspora this is one of our most cherished pictures and I would like you to have it.”
The monarch responded by bursting into laughter and graciously accepting the present.