The 23-year-old joined last night on a four-year contract having received a work permit on Thursday after which he passed a medical in London yesterday afternoon.
The Premier League champions begin their title defence against West Brom today with Blues manager Ancelotti delighted that a deal for the Brazilian had been completed.
Chelsea had been in negotiations with Portuguese side Benfica for more than two weeks over the transfer of the midfielder for a fee believed to be in the region of 17m.
Club representatives finally secured a work permit for the Brazil man despite the fact that he has not played the requisite 75 per cent of matches for his country.
Ancelotti said: "He is a fantastic player. He will be the future of the Brazil national team and I hope he will be the future of Chelsea."
Chelsea's decision to replace Guus Hiddink with Ancelotti last summer proved to be a wise one as the Italian completed a league and cup double in his first year at the helm.
The former AC Milan manager has seen Joe Cole, Deco, Juliano Belletti and Ricardo Carvalho leave the club this summer, with 6m midfielder Yossi Benayoun the only confirmed arrival so far.
To make matters worse, the Blues have endured a poor pre-season, losing four matches on the bounce including a 3-1 defeat at the hands of main rivals Manchester United in the Community Shield last week.
Nevertheless, Ancelotti is determined to build on last year's success – although he does admit that winning the title will be tougher than last time.
"We are ready to defend the title. We have fantastic motivation to win again," the 51-year-old continued.
"I said last year when we won the Premier League and the FA Cup that we had the possibility to open a cycle (of success) and we want to do it.
"The other teams will work harder to beat us but we are ready to fight and we are ready, if possible, to improve our quality, personality and courage.
"The other teams are improving very quickly. Last week's match against Manchester showed that. They played well, better than last year and this means that they are doing better and for this reason we have to have a step up if we want to win."
Chelsea's reported pursuit of Santos striker Neymar took an unexpected twist on Thursday when the Brazilian club's president accused Chelsea of "harassment" over their attempted signing of the 18-year-old.
Chelsea officials said yesterday that they were aware of the claims made by Luis Alvaro de Oliveira Ribeiro but were unwilling to comment on the matter.
Ancelotti said he was unaware of the Brazilian club's complaint.
"I don't know anything about this," said Ancelotti.
"What I can say about Neymar is that he is a fantastic player but he is not a Chelsea player now and it's difficult for me to speak about this."
He added: "We are interested in all the best young players that come through in all the world and Neymar is one of these, but that doesn't mean that we will sign him."
Even with Chelsea completing the 17m signing of Ramires, their net spend of 23m is a drop in the ocean compared to that of title rivals Manchester City, who look set to take their summer spending past the 100m barrier with the transfers of Inter striker Mario Balotelli, which was completed last night, and Aston Villa midfielder James Milner.
Ancelotti has defended his decision not to spend big this summer, insisting that youngsters like Daniel Sturridge, Jeffrey Bruma and Patrick van Aanholt will be able to step up to the plate this year.
"Our aim was to want to refresh the squad with young players and we have done this," said Ancelotti.
"I think that we have a very good squad and we have a new player in Benayoun who has good tactical quality."
West Brom manager Roberto Di Matteo believes he could not have been handed a more daunting introduction to Barclays Premier League management as he prepares to face champions Chelsea at Stamford Bridge today.
As if that was not bad enough for Di Matteo, Albion's following three away games are at Liverpool, Arsenal and Manchester United.
Despite that tough start, Di Matteo, twice an FA Cup winner with Chelsea, is upbeat in his ambition for Albion to get rid of their reputation as a yo-yo club after being promoted or relegated six times in the last nine seasons.
Although only in his third year as a manager, Di Matteo feels that Albion have the ability to avoid relegation and eventually establish themselves as a regular fixture in the English game's top flight.
Di Matteo said: "I want to stay positive and believe we can stay up and remain up in the future. That is my personal aim, the players' aim and the club's aim.
"I am very pleased with the group of players I have. They have been fantastic to me. Last season they gave me everything with effort and commitment.
"But I do believe that we need to add a bit more competition, quality and experience to our team because we cannot dip into the loan market."
Di Matteo, however, fully appreciates the size of his task and is hoping that Albion do not suffer the fate experienced by both Wigan and Aston Villa at Stamford Bridge last season when they both sustained heavy and humiliating defeats at the hands of the champions.
The Italian, making his first return to Stamford Bridge in a professional capacity, admitted: "As they are champions, I guess it is going to be a difficult game for us.
"We will be playing the best team from last season and it is probably the hardest game we could get to start the new season.
"But it would be tough for any club to face Chelsea, Liverpool, Arsenal and Manchester United in your first four away games.
"We are in for a difficult time but the season is 38 games and it will not finish after those four away matches.
"We have to keep in mind that we need to stay positive and to remain balanced when we win or lose."