He won two league championships with the Catalan giants, and was part of the team that beat Arsenal in the Champions League final in Paris in 2006.
However, a series of injuries hampered him during the 2007-08 season, and he made just 14 starts in Barcelona's La Liga campaign.
"I am sure I have come at the right time to do great things and be able to help Chelsea go back to the way it was and come back to being the first one, not only in England but also to win the Champions League that has been their dream for so long," added Deco.
"The Premier League is a great league and it is getting better and you can see that with the Champions League final, where you had two English teams."
Deco's arrival significantly boosts a midfield line-up already featuring the likes of Frank Lampard, Michael Ballack, Michael Essien, Jon Mikel Obi and Claude Makelele.
Deco's arrival looks like hastening the departure of Frank Lampard
BBC football correspondent Jonathan Legard
"It is great that we have so many great players here at Chelsea and it is great that there is the competition which is healthy for everybody and this just makes the team better," he added.
"To have great players in any position of the club just enhances the club. I know it is a new challenge but I am looking forward to it."
BBC Radio 5 Live football correspondent Jonathan Legard said that Deco caught the eye in Scolari's midfield during Euro 2008 more than team-mate Cristiano Ronaldo.
"A two-year deal suggests he is not a long-term building block. He will be 31 in August but his winners' pedigree, Champions League triumphs with Barcelona and Porto will balance recent fears about fitness and consistency," said Legard.
And Legard believes Deco's arrival makes Lampard's possible exit to Inter Milan even more likely.
"Deco's arrival looks like hastening the departure of Frank Lampard, who has done little to quell long-term speculation that he is bound for a reunion with Jose Mourinho at Inter Milan.
"Barring a last-minute change of heart on both sides it's a question of when he signs in Italy, not if."
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