The deal -- a permanent move for the 26-year-old -- is reportedly worth in the region of €30 million, although various performance-related add-ons could be included.
"I'm very happy to be able to play for RB Leipzig again," Werner, who has signed a four-year contract, said. "I had a great time here between 2016 and 2020, when we performed brilliantly as newcomers in the league. It was a dignified departure for me to leave the club as record goalscorer, but that's in the past now and I'm looking ahead, because both I and the club have developed in the past two years.
"I had two great years at Chelsea that I'm really grateful for and were crowned with the Champions League trophy. The experience to play abroad in a new league really helped me and my career. Now I'm looking forward to the new season with RB Leipzig and above all to meeting the Leipzig fans again, who mean a great deal to me. We want to achieve a lot and of course, I want to become the first Leipzig player to reach the 100-goal mark."
Werner will be looking get his career back on track ahead of the World Cup in Qatar after a disappointing two-year spell at Stamford Bridge.
He joined Chelsea in 2020 for €50m, opting not to take part in Leipzig's rescheduled Champions League quarterfinal against Atletico Madrid -- delayed due to COVID-19 -- to give himself more time to adapt to English football.
Werner started well, scoring eight goals in his first 12 games up to mid-November, but managed just four more for the rest of the campaign as a loss of form and confidence set in.
He's back! 💪 pic.twitter.com/aRIhgSK8OV— RB Leipzig English (@RBLeipzig_EN) August 9, 2022
Werner was completely left out of the Chelsea squad for the match against Everton as talks with Leipzig continued.
He has scored 23 goals in 89 appearances for Chelsea and although manager Thomas Tuchel is thought to be reluctant to part with the forward given the wider pressure to strengthen the squad, there is recognition the midseason World Cup has heightened the player's fears over a lack of regular action.
"Timo Werner's signing is a special transfer for us," Leipzig technical director Christopher Vivell said. "We had a lot of great years together in Leipzig that saw Timo become the club's record goalscorer and a Germany international. His return means a lot to the supporters because Timo was a real fan favourite and figure of identification for a lot of people.
"The transfer made a lot of sense to us, both because of Timo's quality and the financial aspects of the deal. Now we want to build on the success that we started together and achieve our goals with Timo's help. Not only does that mean qualification to the Champions League, but we also want to get as far as possible in the cup competitions."
ESPN's James Olley contributed to this story.