The Welsh defender made just five senior appearances under Maurizio Sarri last season, and now boss Frank Lampard recently confirmed that the plan was for Ampadu to head out on loan to continue his development.
RB Leipzig took to their official website to confirm the news, adding that he will be handed the number 26 shirt during his time in Germany.
Sporting director Markus Krosche said: "We are very pleased to have signed Ethan Ampadu on a one-year loan. Despite being just 18 years old, he has plenty of international experience and can play both in defence and as a defensive midfielder.
"Despite offers from many English sides, Ethan has decided to join us, and we now have another top talent in our ranks that is versatile enough to give our squad even more variation."
On his move, Ampadu added: "I am very happy to be at RB Leipzig and start the season with the team. I want to get to know the team as quickly as possible and integrate myself.
"Ive been following the club in recent years and Ive seen how fantastic young players have come to develop here and what potential there is in this club. So its the perfect step for me."
Chelsea took to their website to wish Ampadu luck on his move, with Frank Lampard adding: "I really wanted to work with Ethan this year. That’s a slight disappointment, but it’s to benefit of him. In midfield and defence we have a lot of quality, so each player is a case by case."