Romelu Lukaku was the only major signing during the current transfer window, but the Blues have managed to add the LaLiga-winning midfielder to their squad.
The 26-year-old will join the UEFA Champions League holders on loan for the 2021/22 season with an option to make the move permanent.
"I am very excited to start this new challenge with Chelsea. Blues fans, I am one of you now and I can’t wait to wear the shirt, start training and see all of you. See you soon!," the midfielder told the club's official website.
Niguez has been with Atletico since 2014 and has gone on to make 340 appearances while winning a number of titles, including two Europa League titles and last season LaLiga crown.
Joaquín Correa – Lazio to Inter Milan
Lyanco – Torino to Southampton
Toma Basic – Bordeaux to Lazio
Matheus Cunha – Hertha Berlin to Atletico Madrid
Aaron Lennon – free agent to Burnley
Pietro Pellegri – AS Monaco to AC Milan (loan)
Andrea Pinamonti – Inter Milan to Empoli (loan)
Alexander Sorloth – RB Leipzig to Real Sociedad (loan)
Lucas Torreira – Arsenal to Fiorentina (loan)
Pol Lirola – Fiorentina to Olympique Marseille
Ike Ugbo – Chelsea to Racing Genk
Davide Zappacosta – Chelsea to Atalanta
Thierry Small – Everton to Southampton
Ante Coric – AS Roma to FC Zürich (loan)
Xherdan Shaqiri – Liverpool to Olympique Lyon
Florin Andone – Brighton & Hove Albion to Cadiz (loan)
Ian Poveda-Ocampo – Leeds United to Blackburn Rovers (loan)
Brandon Williams – Manchester United to Norwich City (loan)
Percy Tau – Brighton & Hove Albion to Al Ahly
Cristiano Ronaldo – Juventus to Manchester United