The couple, who have been dating for five years, were spotted affectionately kissing and hugging when they attended the Historic Monte Carlo Rally in Monaco.
Dressed in a chic beige overcoat, a navy turtle neck and brown ankle boots, Italian beauty Beatrice held on tightly to Pierre, who sported a smart blue jacket and beige trousers.
The couple appeared decidedly relaxed, with Beatrice taking out her camera phone to capture the moment Pierre and her brother Carlo Borromeo departed for the rally in a vintage 1979 race car.
Next week, Beatrice, 28, and Pierre, 26, are expected to head to the Swiss resort town of Gstaad, where Andrea and his wife Tatiana Santo Domingo will be saying "I do" again, in a religious wedding ceremony.
Beatrice has accompanied Pierre to several royal weddings over the years. The law and economics graduate was the picture of elegance when she stepped out in Provence to see Prince Félix of Luxembourg marry Claire Lademcher and looked equally as stylish for Prince Albert and Princess Charlene's 2011 nupitals.
Pierre and Beatrice at Prince Felix and Claire Lademacher's wedding (left) and
Prince Albert and Charlene Wittstock's nuptials (right)
Reports suggest that Beatrice and Pierre will be part of a host of special guests who will witness Andrea and Tatiana's three-day celebration.
Andrea and Tatiana, who are parents to nine-month-old baby Sacha, will begin proceedings on Friday 31 January with a pre-wedding fondue party at the alpine restaurant Berghaus Eggli.
The following day a relgious ceremony will take place at the church of St. Nicholas of Myra, followed by a farewell wedding brunch on 2 February at the Palace Hotel.