The 26-year-old, capped by England at various age levels, has signed a one-year deal following a trial period with the Blades.
Morrison, who spent last season with Swedish club Östersund, brings top flight experience to Bramall Lane through his time at West Ham United.
"Ravel is an undoubted talent and we are delighted that we have put together a deal that suits all parties," said Wilder. "I have spoken to a number of people about him and it is clear that he has serious ability.
"We are getting a fantastic footballer with a tremendous pedigree and this is a great opportunity for Rav, we believe we can help him get back to the level he wants to be.
"He has done extremely well during his time with us and it was great to have him in Portugal. It wasn't about him coming and being on trial because we knew about his abilities. Taking him with us was basically getting to know him and what he's about, what makes him tick and how he integrates with the players. Trips like what we have been on are invaluable for that."
Morrison missed last Friday's friendly against Real Betis in Portugal with a heel injury. He joins Luke Freeman, Phil Jagielka and Callum Robinson in signing for United ahead of their return to the Premier League.