Princess Märtha Louise wore a TSH metallic silver one shouldered gown for the gala dinner for the wedding of Prince Guillaume, Hereditary Duke of Luxembourg and the Belgian, Countess Stéphanie de Lannoy on October 19, 2012, at the Grand Ducal Palace. She also had the Order of St. Olav- a Norwegian order of chivalry- on her left shoulder. 

She wore her brown hair down with the King Olav's Gift Tiara. The Royal Order of Splendor said that this is Märtha Louise's only personal tiara. 

The couple married in a civil ceremony earlier that day in Luxembourg's Town Hall. They would have a religious wedding on October 20, 2012, at the Cathedral of Our Lady of Luxembourg. 

Princess Märtha Louise attended with her younger brother, Crown Prince Haakon and sister-in-law, Crown Princess Mette-Marit. Her parents, King Harald and Queen Sonja were also in attendance. 

Royals from all over the world were in attendance at the wedding festivities for the heir to Luxembourg's throne. They included, but were not limited to, Queen Margrethe, Prince Henrik, Crown Prince Frederik, and Crown Princess Mary of Denmark, King Willem-Alexander (then Prince) and Queen Maxima (then Princess) of the Netherlands, and Princess Caroline of Monaco.