Chelsea manager Frank Lampard confirmed last week that the teenage Wales international will depart the club on loan for the upcoming season.
Ampadu did not travel with the Chelsea squad for their pre-season tour of Japan in order to sort out his immediate short-term future.
"I am very happy to be at RB Leipzig and start the season with the team," Ampadu said after the move was confirmed.
"I want to get to know the team as quickly as possible and integrate myself.
"I've been following the club in recent years and saw how fantastic young players have come to develop here and what potential there is in this club. So it's the perfect step for me.
"I'm really happy to be wearing the RB Leipzig jersey now. I have heard a lot about the club and am convinced that the way football is played here suits me very well.
Ampadu made just five appearances for Chelsea last season but not a single one of them came in the Premier League.
Lampard is a big fan of Ampadu as he tried to sign him during his time at Derby County, and he has sanctioned the move for the former Exeter City man to gain more first team experience.
Ampadu joined Chelsea from Exeter City in 2017 and has made eight appearances for the Wales national team.
RB Leipzig finished third in the German Bundesliga last season which has guaranteed them a place in the 2019/2020 UEFA Champions League group stages.
