The former Sao Paulo player joined the Blues at the beginning of this season but he was unable to obtain a work visa until he turned 18.
The Brazil Under-17 international had been playing for the youth and reserve teams while waiting for his birthday on January 20.
Goal.com has claimed that the deal to sign the promising young midfielder was reportedly worth a staggering £10million.
Piazon is not expected to join Andre Villas-Boas first team just yet, but this may not be a long way off if he continues to perform as he has been doing so far.
He has scored twice in five games for the reserves and once on six for the youth team, most notably the winner in the FA Youth Cup tie against Doncaster Rovers.
The former Atletico Paranaense trainee is highly versatile and can also play as a forward, as well as in the centre or wide midfield.
The term of the deal is five-and-a-half years, so should keep him at the club until 2017.