The royal matriarch, who shares four children with her husband King Philippe, dressed to impress in an elegant gold and black floral print peplum top and a pair of smart black trousers.
For her mother-daughter date, Eleonore, 15, looked all grown up in a black outfit, wearing her blonde hair tousled in loose waves.
The royals were treated to a performance of Cassandra at the De Munt - La Monnaie Opera House in Brussels.
The opera is a work by the Belgian composer Bernard Foccroulle and brings the climate issue to the opera hall, as it follows Cassandra, daughter of King Priam of Troy, who is able to see the future but is not heard when she tries to share her knowledge. The masterpiece asks who today's Cassandras are, who see the inevitable disasters of our time but are not heard.
Mathilde, 50, and her youngest child looked thrilled to be attending the opera and had the opportunity to speak to Mr Foccroulle before being shown to their seats.
Eleonore also no doubt enjoyed her special outing on a school night. The royal teenager attends the Heilig Hart College, a Dutch language secondary school in the Wezembeek-Oppem municipality, close to Brussels.
She lives with her parents and her older siblings Princess Elisabeth, 21, Prince Gabriel, 20, Prince Emmanuel, 17 at the Royal Palace of Laeken in the Belgian capital.
King Philippe married Mathilde d'Udekem d'Acoz on 4 December 1999 in Brussels, with the bride wearing a coat-dress style gown by Natan.
The king ascended the throne on 21 July 2013 after his father King Albert II's abdication due to health reasons. Philippe is the eldest child of King Albert II and Queen Paola, who are also parents to Princess Astrid, 61, and Prince Laurent, 59.
Over the summer, the Belgian royal palace released some stunning new portraits of King Philippe and Queen Mathilde ahead of the monarch's tenth anniversary on the throne.
The photos showed Mathilde in white tie dress, and in one outfit looking stunning in a sparkling fuchsia Armani Privé gown, complete with the purple sash of Belgium's Order of Leopold. Meanwhile, King Philippe, 63, wore military uniform with his Order.
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