The Argentina international has put pen to paper on an eight-and-a-half-year contract with the Blues that will keep him at Stamford Bridge until 2031.
The West Londoners were strongly linked with Fernandez throughout the winter transfer window.
However, both parties finally reached to an agreement over a payment structure late on deadline day.
The 22-year-old World Cup winner could make his debut for Chelsea against Fulham on Friday.
Chelsea have now splashed more than £300m on new players in January alone, with Mykhaylo Mudryk, Benoit Badiashile, Noni Madueke, Malo Gusto, Andrey Santos, David Datro Fofana and Joao Felix as new arrivals at Stamford Bridge.
OFFICIAL: Enzo Fernández to Chelsea, signed and completed. 🔵🇦🇷 #CFC
Club statement confirm €121m deal plus extra costs — also, River Plate will receive 25% of it.
Enzo has signed contract valid until June 2031. pic.twitter.com/nbnkdNAPrt
— Fabrizio Romano (@FabrizioRomano) February 1, 2023