After featuring in the World Cup with his country, Atsu becomes the fourth arrival at Goodison Park during the transfer window, after Gareth Barry, Muhamed Besic and Romelu Lukaku.
Boss Roberto Martinez had previously said that the 22-year-old's capture was due to be confirmed on Thursday, but a club announcement on Wednesday revealed a deal for the former Vitesse and Porto man.
"I'm very happy to be joining Everton because it's a great club, which likes to play football, and it's a great pleasure to be here," Atsu told the club’s official website.
"Playing in the Premier League has always been a dream for me and I have achieved that dream.
"This is the biggest level for me to play at. I know that it's a very competitive league and I believe with hard work we're going to be successful this season."
Martinez, meanwhile, believes Atsu's quality will capture the imagination of the Everton faithful.
"He's a player who fits in with the culture and the way we want to play," he said. "He's a very gifted technical footballer and he's got the right personality and character to fit in with the group, which is vital.
"He will excite fans because he brings something completely different and a different way of playing to what we've got in the squad.
"He's a left-footed player who loves taking people on and he's got a really good understanding of the game for such a young man.
"He also brings extra competition in a position in which we felt we needed a player full of desire and hunger, who really wants to make things happen in the final third of the pitch.
"For Christian, at 22, to already have played in a World Cup, to have been able to adapt into Portuguese football and into the league in Holland is a really good accumulation of experiences that will be very beneficial, especially with us playing in the Europa League this season."
