Diaby made 34 appearances across all competitions last season for Thomas Tuchel's side, scoring four times and registering seven assists.
Diaby becomes Leverkusen's second signing ahead of the 2019-20 season, following Kerem Demirbay's arrival from fellow Bundesliga side Hoffenheim, though Julian Brandt has left the club to join Borussia Dortmund, and his newly vacated number 19 shirt will be worn by the new signing.
“Moussa Diaby is an extremely quick and technically strong attacker, whose qualities fit perfectly into our playing philosophy.” - Simon Rolfes pic.twitter.com/3STfxK59qT— Bayer 04 Leverkusen (@bayer04_en) 14 June 2019
"The move abroad into one of the best and most balanced leagues in Europe is extremely exciting," Diaby, who has signed a five-year deal at the BayArena, told Leverkusen's official website.
"I am very curious about Germany, the special football here and the great stadiums. And of course it's great to be in the Champions League as well."
Leverkusen finished fourth in the Bundesliga last season but were 20 points behind eventual champions Bayern Munich and endured a horrendous start to the campaign that saw them hover around the relegation zone at one point.
They were also knocked out of the Europa League at the first knockout stage by Russian outfit Krasnodar on away goals after a 1-1 draw in Germany followed a goalless stalemate away.
The additions of Diaby and Demirbay will boost the third most prolific attack in Germany following Brandt’s departure, but they will need defensive additions in order to close the gap to the top, conceding the most goals of any club in the top eight of the league with 52.