The winger moves to Elland Road until January 5 after failing to make a single appearance since Steve Clarke became Albion manager.
Thomas has been troubled by a series of injury niggles, but that has not stopped Neil Warnock moving for a player he has long admired.
The Leeds boss said: "I've tried to sign him five times so it's about time I finally got hims. He is just what I'm looking for and I want him to come here and enjoy himself.
"He's direct and positive when he's attacking and fits the bill as just what we want."
West Brom youngsters Cameron Gayle and Donervorn Daniels are also on the move until 5 January - to Shrewsbuy and Tranmere respectively.