Wales international Ampadu joined Chelsea from Exeter City in 2017 and made his first-team debut in the Carabao Cup tie with Nottingham Forest that September.
However, the versatile centre-back made just five appearances in 2018-19 and was left out of the Blues' squad for their tour to Japan, with new coach Frank Lampard confirming the player was set to leave on loan.
Sporting director Markus Krosche revealed Ampadu turned down offers to remain in England, with the 18-year-old seeing Leipzig as the ideal place to continue his development.
"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," said Ampadu in a statement.
"I've been following the club in recent years and saw how fantastic young players have come to develop here and the potential there is in the club. This is the perfect step for me."
Krosche said: "We are very pleased to have signed Ethan Ampadu on a one-year loan. He is only 18 and already has international experience and can play in both defence and defensive midfield.
"Despite many offers from England, Ethan decided to join us, and now we have another top talent in our ranks that is versatile enough to give our squad even more variation."