The 19-year-old moved to Old Trafford from Penarol last October in a 10 million euros (£9million) deal.
Pellistri has yet to make a competitive appearance for United and spent the second half of last season on loan at Alaves.
The teenager was back in the north west for pre-season training, making three friendly appearances for Ole Gunnar Solskjaer’s side and scoring one goal.
But Pellistri will continue his development in LaLiga next term, with Alaves announcing he has joined them on loan for the rest of the season.
The Old Trafford club said: “Everybody at United wishes Facundo all the best this season and we will be tracking his progress closely.”