The Swedish royal shared some adorable photos of her son to mark the special occasion – and he looks just like his mother.
Young Prince Oscar was joined by the family's cavoodle puppy Rio in one of the portraits taken by Kate Gabor inside Haga Palace, Solna.
WATCH: Crown Princess Victoria's puppy makes cute cameo in the snow
The five-year-old looked smart in a grey patterned jumper with his white shirt collar poking through the top and his brunette hair swept away from his face.
The photos also inadvertently gave fans another peek at the beautiful royal interior, including a gold-trimmed chair, wooden floorboards, a crystal chandelier hanging from the ceiling and giant windows flooding the property with natural light.
Crown Princess Victoria shared photos of Prince Oscar on his fifth birthday
In another wintery snap taken in the grounds of the royal residence, Oscar and his older sister Estelle wore matching blue puffer jackets and warm hats as they sat in the thick snow.
"Today, H.K.H. Prince Oscar's 5th birthday," the caption simply read, and it wasn't long before royal fans left birthday messages.
"Grattis Oscar, beautiful photos," commented one follower, and a second added: "Happy birthday Prince Oscar."
Prince Oscar was born on 2 March 2016 at Karolinska University Hospital in Solna, Stockholm. He is a grandson of King Carl XVI Gustaf and Queen Silvia and is currently third in the line of succession to the Swedish throne after his mother Crown Princess Victoria and older sister, Princess Estelle.
Prince Oscar's sister Princess Estelle recently turned nine
He was baptised at the Royal Chapel of Stockholm Palace in May 2016, wearing the family's heirloom christening gown, which dates back to 1906.
Victoria shared very similar family photos on Estelle's ninth birthday in February. It showed the young Princess grinning for the camera wearing a white top, matching tights and a checked skirt.
Princess Estelle and Prince Oscar are awaiting the arrival of a new cousin this year, as Prince Carl Philip and Princess Sofia are expecting their third child this spring.