The highly-rated 18-year-old broke into the Toffees first team last season and has made 10 appearances in total, but so far this campaign he has played only in the Capital One Cup victory over Leyton Orient.
Manager David Moyes believes the teenager, who signed a new five-year contract in December, would benefit from a spell of regular football but the player remains a key part of the club's long-term future.
"We have been chasing Ross for some time and I'm really pleased to have him on board," Wednesday manager Dave Jones told the club's official website.
"This is as big as the Rodri (Barcelona striker) signing for me because this lad is a real quality player.
"He's energetic, good in the air and a box to box player who can score a goal."