The Argentine called the Blues ‘the best in the world,’ explaining why the move makes sense for his career.
"These types of moments give you the taste, and you want to keep winning. When you get to a final, you want to score goals and win the trophy. This is my mentality,” he told Chelsea's website.
"I have a winning mentality. I have passion and a big personality. It’s a natural fit with a club like Chelsea, which have just won the Club World Cup.
"Chelsea are at the top now, and it’s important to fight for the club like the fans deserve. I watched the Club World Cup, and to join the world champions is special – it means we’re the best team in the world.
"It’s not common to have played a lot of games at this age, but it has helped me a lot. I’m only 21 and I have a lot of experience from these different types of games, stadiums and moments."
Garnacho could make his Blues debut after the international break against Brentford.