"Craig is a big centre-half that we have been watching from afar," Argyle boss Paul Sturrock has said.
"He is a good centre-half with good qualities and you don't play for Manchester United if you're not of a very decent standard.
"I'm very happy to bring the boy on board and it is up to him to get himself in the team."
Cathcart is expected to make his Pilgrims debut in their Championship game against Wolves this weekend.