The Toffees have secured the right-sided attacker for another term after a successful spell last season.
Harrison is delighted to return to Goodison Park and the Premier League, whereas Leeds are playing in the Championship.
The winger told evertontv: “It’s brilliant to be back. Everton is a great club, there is a fantastic group of lads here and I'm excited to get started again with a full pre-season of training.
“Last year, I had a lot of enjoyment at the Club. We had to face a lot of adversity but we were able to continue on and have a good season overall.
“There are a lot of reasons for wanting to return to Everton. I think the Club in general – the fans, the staff, the manager, my teammates – everyone made me feel really comfortable right away and that's somewhere you want to be as a player. It’s a great place to play football.
“The other thing is I know I have a lot more to offer. That's extra motivation for me to go back and prove to myself and to Everton Football Club that I can do more on the pitch. This season I'll be able to have a full pre-season and hopefully hit the ground running.”