The shot-stopper joins United from NWSL club OL Reign, after a previous spell with French Division 1 Feminine club Reims.
Phallon Tullis-Joyce said: “Today has been pure excitement for me. Manchester United is such an historic club, that has already done so much in the women’s game. I’m so honoured to get this opportunity.”
Head coach Marc Skinner said: “Phallon joins us at an exciting time for Manchester United Women, as we embark on our first European season. We are delighted to welcome Phallon to our family.
“Phallon has a wonderful array of talents, and we look forward to seeing her growth at Manchester United.