King struggled with injuries while on loan at Borussia Monchengladbach in the first half of the season.
King will stay on loan at Hull City for the rest of the season.
“It’s a very similar situation to when we brought Fraizer Campbell here,” Hull boss Nick Barmby told the Hull Daily Mail.
“Fraizer came in as an unknown quantity but players can make a name for themselves here and use the loan as a platform.
“We’re delighted to get Josh in. I first saw him play two or three years ago and I’ve kept tabs on him ever since.
“He’s an exciting player. Josh will fit into the way we play and it’s a great chance for him.
“He trained with Man Utd last week doing fitness tests but he gives us another option. He’s very skilful, quick and really good with the ball.
“He can play on the left of a three, the number nine or on the right of three. That gives us great options.”