The 23-year-old, who has signed a four-year deal with the club, only joined Espanyol from Villarreal last summer, making 26 appearances during his debut season.
"I want to apologise to the club, the players and the fans, because I asked to leave even as they did not want me to go," Wakaso told reporters at a press conference.
"It was an opportunity for my future. When I received the offer, I spoke many times to the president to ask him to accept it, it was important to me.
"I am grateful to the club, the fans, the coaches and the players, this year at Espanyol I have greatly improved and I have learned a lot here."
Wakaso has 11 international caps to his name.