The two clubs and the player had reached a consensus over a transfer on 13 June after Leverkusen signed Heung-Min Son from Bundesliga rivals Hamburger SV.
The transfer has now been rubber-stamped with 22-year-old Schürrle signing on the dotted line for the Stamford Bridge club. He joins former Werder Bremen midfielder Marko Marin at the club now coached, for the second time, by Jose Mourinho.
"I am so happy now I have signed and it is an honour for me to play for this club, with this team and for these great fans," Schürrle said on the website of his new employers.
His loss will be offset by the arrival of Son from Hamburg. “We made it clear from the very beginning that Andre Schürrle’s transfer to Chelsea would only be feasible providing we had an adequate replacement,” Leverkusen's chairman Wolfgang Holzhauser explained.
Schürrle, capped 24 times by Germany, netted 18 goals in the 2012/13 season, taking his total to 38 goals in 131 Bundesliga appearances for Leverkusen and former club 1. FSV Mainz 05.