Having represented Spain during this summer's Olympics, the Basque defender has moved to the Premier League where he will join compatriots Juan Mata, Fernando Torres and Oriol Romeu.
The Spaniard has been brought in to help deal with the shortage of options at right-back in the wake of Jose Bosingwa's release earlier this summer.
In fact, prior to the addition of the 22-year-old, only Branislav Ivanovic was capable of playing regularly at right-back.
That means that Azpilicueta is likely to get plenty of game time at Stamford Bridge and could well be the long-term solution to the club's right-back problem.
Despite his young age, Azpilicueta has three full season's worth of experience playing in La Liga for Osasuna, on top of his two seasons with Marseille.
The Daily Mail is reporting that the fee was around £7 million, although whether or not that is accurate remains to be seen. BBC are reporting that Chelsea have not released the fee or the details of the contract.
Chelsea have been very busy in the transfer market this summer. So far, they have completed deals for Eden Hazard, his brother Thorgan, Brazilian Oscar and German winger Marko Marin.
A deal for Wigan's Victor Moses also seems to be nearing completion, with a fee of £9 million agreed.
Despite their successful Champions League campaign last season, Chelsea drastically underperformed in the league, finishing outside the top four.
Owner Roman Abramovich was clearly not happy with this situation and has made seemingly unlimited funds available to new manager Roberto Di Matteo.
With Manchester City and Manchester United dominating the league last season, Chelsea will be leading London's charge to try and sustain a challenge for Premier League glory all the way through to May.
The addition of Azpilicueta could prove to be an important step toward such a challenge, given the previous weakness in the right-back department and squad depth being a key feature of any successful Premier League side.
There will be a lot of pressure on Azpilicueta to adapt to Premier League life quickly. He has a chance of featuring this weekend against last season's surprise package, Newcastle.