"I'm delighted to be a Chelsea player and can't wait to get started," Drinkwater said in a statement.
"It has been a long journey to get here but I am very happy and am looking forward to helping the club win more trophies."
Drinkwater, who came through Manchester United's youth-system, won three England caps since making his international debut in March 2016.
He is Chelsea's sixth signing of the transfer window following the arrivals of Willy Caballero, Antonio Rudiger, Tiemoue Bakayoko, Alvaro Morata and Davide Zappacosta, who also signed on the transfer deadline day.
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(Reporting by Hardik Vyas in Bengaluru; Editing by Ian Ransom)
