The 20-year-old Brazilian spent last season at Vitesse Arnhem, where he scored 11 goals in 29 games.
While claiming that he is happy to be back at Chelsea, Piazon does not know where he will be playing during the upcoming campaign.
"I don't know what is going to happen but at the moment I'm really happy to be here. I will play in pre-season until we make a decision and I will try to do what's the best thing for me," he told the club's official website.
Piazon signed his first professional contract with Chelsea in 2012 following a spell in the club's youth system.