The 16-year-old, who trained with the senior Wales squad ahead of Euro 2016, has signed a deal with the Premier League champions after deciding to leave Exeter, the Sky Bet League Two club have announced.
Exeter will not receive a transfer fee as Ampadu had yet to sign scholarship forms at St James Park but the Grecians are entitled to compensation for their academy product.
Exeter said in a statement they hope a fee can be agreed with Chelsea for the Wales Under-19 defender without the need to go to a tribunal.
Ampadu became Exeter's youngest debutant - beating Cliff Bastin's long-standing record - after being given his bow in last season's League Cup match against Brentford aged 15 years, 10 months and 26 days, and he went on to make 13 appearances for the first team
Exeter manager Paul Tisdale said on his club's website: "Everyone at Exeter City fully supports Ethan's decision to sign for Chelsea.
"He had many options available to him and has decided that Chelsea provides him with the best opportunity to develop the next step of his career
We wish him the very best of luck."
Source : PA
Source: PA