On 31 August 2017, he was sold to Chelsea for a reported fee of €28 million plus bonuses. He signed a four-year deal at Stamford Bridge. The transfer was completed minutes before the transfer window shut on deadline day. Zappacosta made his debut for Chelsea on 9 September as a substitute replacing Victor Moses in the 74th minute of a 2–1 away win against Leicester City. He made his Champions League debut and scored his first goal for Chelsea on 12 September 2017, in a 6–0 home win against Qarabağ FK. With this goal, Zappacosta became the 100th Italian player to score a goal in the Champions League. His first Premier League goal came from outside the box in a 5–0 home win against Stoke City on 30 December 2017. At the end of the season he was an unused substitute as Chelsea beat Manchester United in the 2018 FA Cup Final.