Prince Harry played host to three generations of the US First Lady's family at Kensington Palace as he welcomed Michelle Obama to the UK.
Mrs Obama, her teenage daughters Malia, 16, and 14-year-old Sasha and her mother Marian Robinson headed to Harry's London home to take tea with the fifth in line to the throne.
The First Lady and her family spent around 40 minutes chatting with Harry, who was keen to catch up with them after the warm welcome he received at the White House during his trip to America two years ago.
Kensington Palace said the prince was "very pleased" to host Mrs Obama and "delighted" to meet her daughters and her mother, who is said to be known as Mrs R.
Harry is known to get on famously with Mrs Obama. He was a surprise guest at her Mother's Day tea for military mothers in the US in 2013. .
Mrs Obama is in the UK to promote her campaign for girls' education and better support for military families.
A Kensington Palace spokesman said: "Prince Harry was very pleased to host The First Lady of the United States Mrs Michelle Obama for tea at Kensington Palace this morning.
"They discussed their shared interest in support for veterans and their families. Prince Harry was also interested to hear from the First Lady about the Let Girls Learn initiative to further adolescent girls' education around the world, which is the focus of her visit to the United Kingdom.
"Prince Harry was also delighted to meet Malia and Sasha Obama and The First Lady's mother Mrs Marian Robinson."
The First Lady's Joining Forces campaign works with the public and private sectors to help service members, veterans and their families with employment, education and well being.
Mrs Obama, who will also meet with Prime Minister David Cameron and his wife Samantha, went on to host an event at the Mulberry School for Girls in Tower Hamlets in east London to discuss joint work between the two nations to boost education for adolescent girls across the world through the Let Girls Learn initiative, championed by her and her husband, US president Barack Obama.
Writing in the Financial Times, Mrs Obama cited figures suggesting more than 62 million girls were out of school across the world, which she described as "a heartbreaking injustice".
Girls faced obstacles to education such as forced marriages, early pregnancies, abuse and sexism, she said.
"That kind of life is unthinkable for the girls in our lives, so why would we accept this fate for any girl on this planet?" she wrote. "This week I will join Prime Minister David Cameron in London to begin to answer that question, and announce a series of partnerships between the US and UK to educate adolescent girls in developing countries around the world."
She praised the UK as a "global leader" for girls' education and said one joint scheme included a 180 million dollar (£116m) investment in the Democratic Republic of Congo.
She added: "In addition, our development agencies and two of countries' leading universities will collaborate on evidence-based research to determine the best ways to educate adolescent girls. And British and American partners will work together to support teacher training, girls' leadership camps, and other community-based programmes in developing countries.
"Combined, these efforts total nearly 200 million dollars (£128m) - but, given the scope of this challenge, even that is nowhere near sufficient. Girls' education is a global issue that requires a global solution."
After London, the party will fly on to Italy to meet US armed services families based in Europe and to visit the Milan Expo as part of the third strand of her work, encouraging healthier diets for children.