The midfielder will stay at Elland Road for one month in the hope of securing a run of first-team football.
The 19-year-old has already had a loan spell at Sheffield Wednesday this season, playing 13 times and scoring four goals.
Barkley has made just two appearances for the Toffees this current campaign both as a second half substitute.
Warnock insists Barkley, who he has wanted to bring to the Whites for some time, had options away from Elland Road.
"I'm absolutely delighted to get Ross and I'd like to welcome him to the club," 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 could make his Leeds debut in Saturday's local derby against Barnsley at Oakwell.