The Championship club also have the option of completing a permanent deal for the 24-year-old at the start of next year.
Leeds manager Neil Warnock has revealed that he was a big fan of Hall when he was manager at Crystal Palace.
"I've known Ryan for many years and was a big fan of his at Palace," Warnock told the club's official website.
"I don't know if he can do it at this level yet but he offers something different in the squad."
Hall scored 19 goals in 84 appearances for Southend United.