The 26-year-old, previously of Crystal Palace and Millwall, has agreed a one-month deal.
“This is a great acquisition for us,” Saddlers manager Dean Smith told the club’s official website.
“He’s a player that I have been aware of since his Millwall days. He gets plenty of crosses in, scores his fair share of goals and also possesses good pace.
“He played against us last season while on loan at Notts County and also last week for Derby reserves, so I know what he can do and with Claude Gnakpa out injured, he’ll give us another option out wide.
“I’m looking forward to working with him here.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.