The 18-year-old striker has signed on the dotted line after leaving Barcelona.
The Spanish giants had hoped to retain Guiu’s services, but a five-year deal from Chelsea was too good to turn down.
Speaking to club media through a translator, Guiu said: “It brings me immense joy to sign for Chelsea. I struggled to sleep last night ahead of the journey here because I was so excited.
“Ever since I was small, it was my dream to play in the Premier League and be successful in England. Now I have the opportunity to join Chelsea and I will work my hardest to be successful for the club.
“I think what's most important is that I adapt to my surroundings, which I'm sure will be easy, and then of course I want to win the Champions League and the Premier League for Chelsea.
“On a personal level, I want to help the team as much as I can by scoring lots of goals and I want to become the best center forward in the world.”