Marc Guiu, who is only 18, has put pen to paper on a six year contract at Stamford Bridge.
Guiu, who is a forward, will be hoping to make his mark in the Premier League after leaving La Masia.
In an Instagram post, he wrote: "The truth is that I can only have words of thanks for the club that trained me.
"For this, I would like to thank the entire Barca family, coaches and coordinators, who have helped, taught and supported me so much.
"To the physiotherapists and doctors, for the attention and express recoveries.
"To the staff, educators, taxi drivers for taking care of me and for passing on values that I will always carry within me.
"To the fans who encouraged me from the first day, thank you all; and also especially to each and every one of my colleagues and friends, for so many smiles and so many games we won together, some of them will always be my family.
"I will be supporting you from a distance. Now and always, long live Barca!"