The 18-year-old, who joined Chelsea from Exeter City in July 2017, made seven first-team appearances for the Blues during the 2017-18 campaign.
The teenager, who is also a three-time Wales international, has revealed that he is "really proud and happy" to have extended his contract at Stamford Bridge.
"I'm really proud and happy. I'm looking forward to the next five years and all I have got to do now is continue to work hard, and hopefully good things will happen," Ampadu told the club's official website.
Chelsea director Marina Granovskaia added: 'Ethan has shown great potential and unbelievable maturity since arriving at Chelsea a year ago.
"In this short time he convinced a lot of people with his exceptional quality and mentality and we look forward to watching him develop into a key player for the club over the coming years."
Ampadu is yet to feature for Chelsea in the early stages of the 2018-19 season.