The 18-year-old Chile Under-23 striker had a medical with the Red Devils last month and has now joined from Universidad de Chile after being granted a work permit.
“I am thrilled to be joining the biggest club in the world. Manchester United is a great team and has a great manager,” he said.
“To be able to be part of that is a huge honour and I can’t wait to get started.
“I hope I can be part of many successful years with United.”