And Koeman is confident the England Under 21 international will fulfil his potential.
"I'm really pleased we've made what I'm sure will be a significant signing for Everton," said Koeman.
"Jordan is a very talented young professional who also has a real hunger for success, and that's important for us as we now look to make further progress in the seasons ahead.
"We've seen the quality he has through his performances in the Premier League last season and, given his young age, we're sure he will get even better.
"I'm looking forward to welcoming Jordan to the group next month and wish him every success with the England U-21 squad in Poland."