The 26-year-old arrived in England on Wednesday, January 28 and his signing was confirmed on Friday
"I am so happy to sign for Chelsea. It is a dream for me. I am looking forward to meeting and getting to know my new team-mates and cannot wait to play," he told Chelsea club's official website.
"I thank Chelsea for their support and hope I can repay this faith to the club and its fans."
The 26-year-old netted 18 goals in 47 league appearances for Sao Paulo on loan from Corinthians in 2015, and arrives as a welcome boost to Chelsea's forward line.
Pato was widely thought of as one of the game's hottest properties following his move from Internacional to AC Milan in 2007.
Enjoy this video of Alexandre Pato , below: