Princess Beatrice's wedding to Eduardo Mapelli Mozzi will take place on 29 May, Buckingham Palace has announced.
They will be married at the Chapel Royal at St James Palace, with a reception later at Buckingham Palace.
The Queen will host the private reception in the palace gardens.
Beatrice, the 31-year-old daughter of the Duke of York and the Duchess of York, became engaged to her property tycoon boyfriend in Italy last September.
Buckingham Palace says that the choice of the Chapel Royal, a family chapel adjacent to the home of Princess Beatrice, is an indication of the scale and nature of the wedding.
It has been chosen by Beatrice and Mr Mapelli Mozzi, known as Edo, for its "intimate atmosphere". The chapel is also where the wedding of Queen Victoria and Prince Albert was held in February 1840.
The announcement ends weeks of speculation over the date of the wedding, amidst the scandal over Beatrice's father, Prince Andrew's relationship with convicted sex offender Jeffrey Epstein.
Prince Andrew has retired from royal duties for the foreseeable future.
While the wedding of Beatrice's sister, Eugenie, was shown on ITV in 2018, there is no intention to televise this ceremony, said Buckingham Palace.
No public money is to be spent on the event, which is set to be more of a low-key affair.
It will be the fourth royal wedding in two years. The previous three - Princess Eugenie, the Duke and Duchess of Sussex and Lady Gabriella Windsor - all took place at St George's Chapel in Windsor Castle.
Beatrice's wedding venue can hold 150 guests, while around 800 attended her sister's wedding.
Millionaire Mr Mapelli Mozzi, who has been a friend of Beatrice's family for several years, started dating the princess in autumn 2018. Their relationship is said to have started after they met each other again at Eugenie's wedding.
Beatrice, a cousin of the Duke of Cambridge and the Duke of Sussex, is ninth in line to the throne. She is not a full-time royal but works for artificial intelligence software firm Afiniti, where she is vice president of partnerships and strategy.
At the time of their engagement, the couple said, in a joint statement: "We are both so excited to be embarking on this life adventure together.
"We share so many similar interests and values, and we know that this will stand us in great stead for the years ahead, full of love and happiness."
His parents, Nikki Williams-Ellis and former Olympic skier Count Alessandro Mapelli Mozzi, said at the time of the engagement that the couple was "made for each other" and that the family had "known Beatrice for most of her life".
Mr Mapelli Mozzi, who is also a count, has a young son, Wolfie, from a previous relationship.