The 19-year-old signed for the club in the summer of 2020 and has made a series of impressive appearances for our Under-23s side over the last few months.
He will now head to La Liga in a bid to gain regular first-team experience and make the next few steps in his development.
Pellistri has a five-year contract with United, with the option to extend for a further year, and Ole Gunnar Solskjaer has said he believes the youngster has a bright future.
Upon his signing, the Norwegian said:
“Our scouts have followed Facundo for a long time and we believe that he will be a real talent for the long term.