The 21-year-old has put pen to paper on a massive long-term contract that will keep him at Stamford Bridge until 2032, marking a significant coup for Blues manager Enzo Maresca.
"It's an incredible moment for my family and I to join this great club," Garnacho told Chelsea's official website. "I can't wait to get started. I watched the Club World Cup and to join the world champions is special – we're the best team in the world!"
The transfer ends a prolonged period of speculation surrounding Garnacho's future at Old Trafford. Tensions reportedly boiled over after he was left out of the starting lineup for last season's Europa League final, leading to a critical social media post from his brother and public questioning from the player himself. United's manager, Ruben Amorim, who prefers a system without traditional wingers, subsequently informed Garnacho he could find a new club.
Chelsea, who had identified a left winger as a key priority, moved decisively to secure their long-term target. The deal is seen as a piece of shrewd business for a player of Garnacho's age and profile, though his time at United was marked by questions over his efficiency in front of goal. Last season, he converted just two of the 16 clear-cut chances he was presented with.
Garnacho becomes Chelsea's ninth signing of the summer and the fifth addition to their attacking ranks, joining a forward line that already includes new arrivals Joao Pedro, Liam Delap, Estevao, and Jamie Gittens. He will provide direct competition on the left wing as Chelsea looks to defend their Premier League and Club World Cup titles.