On 8 August 2018, Kovačić joined Chelsea on a one-year loan. He made his debut in a 3–2 home win against Arsenal on 18 August, appearing as a second-half substitute. On 29 May 2019, he was named a starter for 2019 UEFA Europa League Final in Baku, playing until 78th minute when he was substituted off for Ross Barkley, as Chelsea defeated Arsenal 4–1.