They went as guests of President Mário Soares and First Lady Maria Barroso in 1987.
When they arrived in Lisbon, the royal couple were welcomed in Lisbon by a ceremony which was followed by a Gala Dinner at the Ajuda Palace.
The couple also visited the town of Sintra.
When they were there, they walked on capes which were thrown onto the ground by students from the Oporto University.
There’s even a video of Diana seen walking on the capes.
The trip to Portugal marked the first “public clue” that there was a lot of “trouble” in the couple’s marriage, according to a royal expert.
Royal biographer Sally Bedell Smith made the claim in a series called The Windsors: Inside the Royal Dynasty.
She said: “The first public clue that there’s a lot of trouble in the marriage was when they went to Portugal and the press sussed out that they were staying in separate suites.”
The series is a six-part CNN documentary which aired earlier this year.
The documentary explored family dramas and scandals which have rocked the Royal Family.
The documentary included interviews with insiders and royal experts who examined the famous family.
Anne Allan, Princess Diana’s former dance teacher also appeared on the series.
She said: “The press is beginning to get wind of the truth of what’s happening.”
Biographer Andrew Morton added: “I remember writing a story for my newspaper saying they had separate bedrooms, separate schedules, separate lives.
“That sparked a while lot of focus on the royal couple.
Mr Morton said in an interview at the time in 1987, “Charles has been off to Italy sketching, he’s been off in Southern Africa trecking, in Scotland fishing, farming, doing almost anything to stay away from Kensington Palace.”
Journalist Richard Kay said: “We saw Charles and Diana but not often together and it was increasingly clear they were spending an awful lot of time apart.”
Charles and Diana married in the summer of 1981.
Their landmark royal tour was of Australia and New Zealand in 1983.
It was Princess Diana’s first ever trip overseas.
Source: EXPRESS CO UK