The 26-year-old Portuguese has become the Blues’ seventh fresh summer signing, joining for a £16m fee from recently-relegated Championship club Burnley, and penned a five-year deal at Portman Road.
“It feels brilliant to be a part of this club,” said the former Benfica man.
“I think I will be very happy here. I like how the club sees the future and there are a lot of good players here. How the club is thinking to the long-term is one of the main reasons I came here.
“I can bring experience and balance to the team, between defence and attack, and I like to bring the connection.
“Now I have signed I can’t wait to meet my teammates and the fans and get started with the team.”
Blues boss Gary O’Neil said: “We are delighted to have added Florentino to the group.
“He is a player who brings experience of playing in the Premier League as well as other major competitions in Europe, including the Champions League, and makes us stronger in central midfield.
“He brings quality off the ball, as well as on it, and we look forward to him joining up with the squad.”