The 18-year-old Wales international of Ghanaian descent is yet to feature under new head coach Maurizio Sarri, but made seven appearances under his predecessor Antonio Conte last term.
Speaking to Chelsea's official club website, he said: "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."
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."
Asked last week what Ampadu has to do to break into the Chelsea starting XI, Sarri said: "In this first period, for me it was very important not to change especially the defensive line.
"But I hope in the future to be able to give space to everyone, even the defenders. In the first period, as you know, it's very difficult to change the defensive line."
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Cardiff boss Neil Warnock said, ahead of Saturday's 4-1 defeat at Stamford Bridge, that he would be keen to take Ampadu on loan, saying he "doesn't look out of place" at the top level.
Ampadu announced himself on the international stage during his competitive Wales debut - a man-of-the-match display in a 4-1 victory over Republic of Ireland - leading to calls for him to make the Blues first team.
Source: Wires
