King Frederik and Queen Mary's second child, who will officially mark her milestone age on 21 April, was the image of her mother in a pale blue power suit.
The event at Aarhus City Hall saw musical performances, dancers and a fashion show organised by local youth talent.
It was previously confirmed by Aarhus Municipality that the budget for the celebration cost 290,000 kroner (£32,542), including 63,000 kroner (£7,075) for catering and drinks for 300 guests.
EURO ROYALS
As well as her parents, Isabella was also joined by her younger siblings, 14-year-old twins Prince Vincent and Princess Josephine.
The only missing family member was King Frederik and Queen Mary's eldest child, Crown Prince Christian, 19, who is currently undergoing military training with the Grand Hussar Regiment at Antvorskov barracks in Slagelse.
A large crowd gathered outside Aarhus City Hall to greet the Danish royals. Princess Isabella, who looked beautiful in a pale blue suit by one of Queen Mary's go-to designers, Soeren Le Schmidt, smiled and waved at members of the public.
The Princess was also given a posy of bright red flowers.
Meanwhile, Mary, 53, and her youngest daughter, Josephine, matched in white coats, while Frederik, 56, and his son, Vincent, both sported suits. Hasn't he got tall?!
During the engagement, Isabella delivered her first official speech, recalling her fond childhood memories from Aarhus.
"It makes me really happy to be celebrated in a city that has great meaning to me and my family," she said. "Here I have celebrated Christmas and Easter, taken walks in the garden and by the water, and enjoyed myself in the Old Town since I was little."
She continued: "I have many good memories from Aarhus, today I have another. And there will definitely be more after that.
"Thank you for the dance and music, the show and the cake. I have pursued many hobbies myself, and it has been a shortcut to joy, friendship and community for me. I am sure it is for you too.
"Thank you for the birthday song, a whole book of greetings and for the beautiful piece of jewellery."
The fact that you have chosen to celebrate me, and that so many have come together, makes me happy and touched. You should know that it means a lot to me.
The royals reacted as acrobats and gymnasts unveiled a special birthday banner on the roof of City Hall as the event concluded.
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