The 22-year-old arrives for an undisclosed fee believed to be in the region of £17m.
Mykolenko’s arrival casts doubt over the future of France international Lucas Digne, who has fallen out of favour with manager Rafael Benitez and has been linked with Chelsea.
Mykolenko has signed a four-and-a-half-year deal at Goodison Park.
Speaking to Everton's official website, Mykolenko said: "Moving to Everton is a big and important step in my life.
"I have always dreamed of playing in the Premier League. English football is where the game was born and I think that it will suit my game very well.
"I am 22 but I don't feel like a youngster. I have experience of playing at European level and hope I will continue to improve as a player.
"I am excited about the opportunity of playing in front of our supporters at Everton. I have seen the last few games at Goodison Park and they were unbelievable.
"They are incredible fans to whom I feel great responsibility."
Mykolenko has won 21 caps for the Ukraine national team and was part of the side that reached the quarterfinals of EURO 2020.
