Sonja became engaged to then Crown Prince Harald in March 1968. They had been dating for nine years, although their relationship had been kept secret because of her non-royal status. The Crown Prince made it clear to his father, King Olav V, that he would remain unmarried for life unless he could marry her. This would in effect have put an end to the rule of his family, and likely to the monarchy in Norway, as Harald was the sole heir to the throne. Faced with having to choose one of his relatives from the Danish Royal Family, the Dukes of Schleswig-Holstein or even the Grand Dukes of Oldenburg as his new heir in place of his son, Olav V consulted the government for advice and, as a result, the couple was wed on 29 August 1968, at Oslo Domkirke in Oslo. She thus acquired the style of Royal Highness and the title of Crown Princess of Norway

29th August 1968. The long-awaited day had finally come: Like most of the Scandinavian royal weddings, the wedding of Crown Prince Harald and Miss Sonja Haraldsen had a special touch of glamour, set by the gala attires and jewels worn by women to the ceremony and subsequent banquet and ball at the Royal Palace. The streets of the Norwegian capital were decorated as they had not been for many years: Norwegian flags and wedding banners and other decorations relating to the couple had flooded Oslo in the days prior to the wedding.