The 26-year-old Tullis-Joyce previously played in Europe with French club Reims. United did not specify the length of the contract or value of her transfer.
“Today has been pure excitement for me. Manchester United is such a historic club, that has already done so much in the women’s game. I’m so honoured to get this opportunity,” she said.
England international Mary Earps is United’s current first choice goalkeeper. She has been linked with a move to Women’s Super League rival Arsenal after winning the Golden Glove at this year’s Women’s World Cup but will remain at United until at least the January transfer window.
Tullis-Joyce is initially likely to provide back up to Earps.
“Phallon has a wonderful array of talents, and we look forward to seeing her growth at Manchester United,” United coach Marc Skinner said.
Reporting by The Associated Press.
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