Drogba, 36, spent eight seasons at Stamford Bridge before leaving in 2012 shortly after scoring the winning penalty in the Champions League final against Bayern Munich.
“It was an easy decision – I couldn`t turn down the opportunity to work with Jose (Mourinho) again,” Drogba told the club website.
“Everyone knows the special relationship I have with this club and it has always felt like home to me.”
Drogba joined the Blues from Olympique Marseille for 24 million pounds ($40.8 million) in 2004 and scored 157 goals in 342 appearances for the London club.
He is Chelsea`s record scorer in European competition with 34 goals and won three Premier League titles, four FA Cups, two League Cups as well as the Champions League in his first spell.
After leaving Stamford Bridge two years ago, he moved to China to play for Shanghai Shenhua before joining Turkish side Galatasaray, who played Chelsea in the Champions League last-16 last season.
Chelsea coach Mourinho said there was no emotion or sentiment involved in the decision to bring him back.
“He’s coming because he’s one of the best strikers in Europe,” Mourinho said.
“I know his personality very well and I know if he comes back he’s not protected by history or what he’s done for this club previously. He is coming with the mentality to make more history.