"It was an easy decision - I couldn't turn down the opportunity to work with Jose again," Drogba said. "Everyone knows the special relationship I have with this club and it has always felt like home to me."
Drogba became a free agent upon leaving Galatasaray at the end of last season so there is no transfer fee involved in the deal. The Côte d'Ivoire marksman was named Chelsea's greatest ever player in a poll of fans in 2012 after scoring 157 goals in 341 appearances.
Mourinho, who originally lured Drogba to Chelsea for £24m in 2004, insists the decision to re-sign the club's fourth highest scorer of all time was not clouded by sentimentality. "Didier'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."
It was Drogba's equalising header that took the 2012 Champions League final into extra time and his 34 goals in European competition are a Chelsea record, as are the nine strikes in nine cup finals. The first spell at Stamford Bridge, spanning eight years, produced three English Premier League titles, four FA Cups, two League Cups and the prized victory over Bayern Munich.
Drogba's spell in Chinese football failed to last one of the two-and-a-half year deal, but he enjoyed greater success at Galatasaray, winning the 2012/13 Turkish league title. At Chelsea he joins a crop of strikers including new signing Diego Costa as well as Romelu Lukaku and Victor Moses, who both spent last season out on loan. Samuel Eto'o and Demba Ba have already departed the club this summer.