The 27-year-old defender joins the club from Chelsea, and has also previously played for Klepp in the Norwegian top division.
Thorisdottir is an experienced international player, with 46 caps for her country, and was a key part of the Norway squad who reached the quarter-finals of the 2019 Women’s World Cup in France.
Maria Thorisdottir said:
“I’m proud to be joining Manchester United. Growing up, the club was always big in Norway, so for me to now be a part of the women’s side is really exciting.
“I’ve been impressed with the way the team have developed in a short space of time and have heard so many good things about Casey [Stoney] too, so I can’t wait to start working with her and the rest of the staff.