The 19-year-old will stay at Elland Road for an initial month.
"I'm absolutely delighted to get Ross and I'd like to welcome him to the club," manager Neil Warnock told the club's official website. "He had four other clubs after him so I'm delighted he's come to us.
"I've wanted to sign him for a long time now, I remember watching him when he played for Everton against QPR in the Premier League.
"He's got a great attitude and I have no doubt that he will end up a top player.
"The deal works well for both parties as he will gain experience here and he is a great addition to our squad."
Barkley, a product of Everton's academy, signed his first professional deal with the Merseysiders when he was 17.
He spent two months on loan at Sheffield Wednesday earlier this season and caught the eye with some outstanding displays in the centre of midfield.
Barkley, who made 13 appearances for Wednesday, scoring four goals, will go straight into Leeds' squad for Saturday's derby clash at Barnsley.
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