The 24-year old winger has landed at Elland Road on an initial three-month loan with the view to a permanent move.
"I've known Ryan for many years and was a big fan of his at Crystal Palace," said White boss Neil Warnock.
"All credit to him to come back into my thoughts. I'm looking for a player of his quality. I don't know if he can do it at this level yet, but he offers something different in the squad. He can give us an option where we could change a game."