The Blues have an option of making the move permanent at the end of the season for a fee of 7m.
The Brazilian striker completed his eleven and a half hour journey from Sao Paulo to London yesterday and was spotted at the Heathrow airport.
Pato confirmed that he was signing for Chelsea and expressed his delight after having agreed on the move.
He said: “So happy, so happy, I stay here for my dream. Chelsea is my new home ‘Thanks Chelsea for the support. Thank you for my fans.