Leverkusen have moved to quickly to replace the departed Heung-Min Son, bringing in midfielder Kevin Kampl from Bundesliga rivals BVB Dortmund.
The 24-year-old is no stranger to the surroundings at BayArena, having spent three years in the club's youth system before departing for VfL Osnabrück in 2011.
Kampl represented the likes of VfR Aalen and Red Bull Salzburg prior to linking up with Dortmund in December 2014 on a five-year deal.
"I am looking forward to returning to Leverkusen, where I made my first important steps as a young player in Bayer's Under-23s," Kampl told the club's official website.
"To now link up with the professional team in the Bundesliga and the Champions League is great."
Dortmund, meanwhile, posted a brief statement on their website, with sports director Michael Zorc saying: "We wish Kevin all the best for his future sporting career."