Ari Behn, Princess Martha Louise's husband for 14 years, told Norwegian radio that Spacey, who sat next to him, suddenly said "'hey, let's go out and have a cigarette'", before "he touched me" on the genitals.
Mr Behn said that he declined Spacey's approach by saying "maybe later".
The 45-year-old married Martha Louise, fourth in line to the Norwegian throne, in 2002.
Last year they decided to split but share custody over their three daughters.
Spacey, who was co-hosting the December 11 2007 event, has faced numerous sexual misconduct and assault allegations, but he has remained mostly silent.