On 5 January 2018, Barkley joined Premier League club Chelsea for a fee of £15 million, signing a five-and-a-half-year contract with the defending champions. He was assigned shirt number 8 by the club, the same number he wore at Everton. Barkley had been close to joining Chelsea previously on the summer transfer deadline day, before a move failed to materialise due to him suffering a long-term injury and wanting to consider his options. Days after its completion, the transfer was referred to Merseyside Police by the Mayor of Liverpool, Joe Anderson, to investigate the possibility of fraud in the transaction. No fraud was found. Barkley made his debut in a 2–1 defeat at Arsenal in the semi-final second leg of the EFL Cup on 24 January. He came on as a first-half substitute, replacing the injured Willian. Barkley made his Premier League debut in a 3–0 home loss to AFC Bournemouth. At the end of the season he was an unused substitute as Chelsea beat Manchester United in the 2018 FA Cup Final.