Benfica midfielder Tiago has completed his move to Chelsea after agreeing personal terms with the London club.
The 23-year-old, who had already passed his medical, has agreed a three-year deal at Stamford Bridge with the fee thought to be about £10m.
Tiago was contracted at Benfica until 2007 but left due to differences with the club's new chief Jose Veiga.
"There are big players in my position, but I go to work - then the coach Jose Mourinho will decide," said Tiago.
His signing ends Jose Mourinho's search for a midfielder after the new coach failed to sign Steven Gerrard.
The transfer had been in doubt after Mourinho admitted negotiations over personal terms had hit a stumbling block.
The Portuguese international, a member of the country's Euro 2004 squad, follows Mateja Kezman, Paulo Ferriera, and Didier Drogba as Chelsea's summer signings.
Tiago has scored 19 goals in 95 matches for Benfica after joining them from Sporting Braga in 2002.
Mourinho had insisted any new signings must be completed by Wednesday, when the squad leave for a tour of the United States.
However, Tiago is set to miss the opening games of the season after being called into Portugal's squad for the Olympics in Athens.
The squad will meet on 1 August before flying to Greece on 8 August after a
friendly with Paraguay in Faro.
Portugal are drawn with Iraq, Morocco and Costa Rica in Group D. The
tournament begins on 11 August and finishes on 28 August with the gold medal