The 21-year-old full-back joins the club following the conclusion of her contract at Spanish side Levante, where she has spent the past two seasons.
Batlle spent her youth career at Barcelona and has also played for Madrid Club de Fútbol.
Ona Batlle said:
“It’s an honour and a pleasure to sign for Manchester United; to work with Casey and be a part of this team is a fantastic opportunity. To have the chance to come over to play in England and really push myself as a player is very exciting. I can’t wait to meet my new team-mates and help to contribute to as much success as I can.