Howard, 25, who has been on trial at Bramall Lane following his recent release from Blackpool, has put pen to paper on a two-and-a-half year contract.
He told the club’s official website: “I’m very pleased to be joining a club the size of Sheffield United. I’ve really enjoyed training with the boys over the last couple weeks and was really keen to sign.
“The Blades are playing very well and pushing for promotion so it promises to be a very exciting run-in.
“I know how well Steve Simonsen is playing and how highly he is thought of at this club so I’m under no illusions about how hard it will be fighting for a first-team place.”
Manager Danny Wilson added: “Mark will give us back-up and experience. You can never have too much cover. I hope nothing happens to Simmo, but you can never be sure what’s round the corner.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.