The 26-year-old midfielder has spent the majority of her playing career in Norway and Sweden, with spells at Arna-Bjørnar, Göteborg FC and Sandviken respectively in recent years.
Bøe Risa has 37 caps for her country; she made her senior debut for Norway in 2016 and was a key part of the squad that reached the quarter-finals of the 2019 Women’s World Cup in France.
Vilde Bøe Risa said: “I’m happy to be here, at such a big club that I have grown up supporting. Manchester United is huge in Norway so to be able to come here and play for the team now is a dream come true.