World Cup Golden Boot winner Miyazawa joins Manchester United
Japanese midfielder Miyazawa scored five goals in Australia and New Zealand as her team reached the quarter-finals, before losing to Sweden.
She was the first non-European player to win the Golden Boot at the tournament.
The 23-year-old last played for MyNavi Sendai, who finished fourth in Japan's WE League last season.
"I am truly very happy to be part of this wonderful family and this great team," Miyazawa said in a statement released by the club.
"I am really looking forward to getting started and hope my style of play will excite our fans."
United coach Marc Skinner said Miyazawa had "proven her qualities at the highest level".
"As her Golden Boot win at this summer's Women's World Cup has shown, she will excite our fantastic fans with her direct style of play and her ability to find the most dangerous spaces.
"We cannot wait to see her develop even further within our playing squad."