The 36-year-old striker and Blues favourite returns to Stamford Bridge after leaving in 2012 for stints at Shanghai Shenhua and Galatasaray.
"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.
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The Ivorian marksman was named Chelsea's greatest ever player in a poll of fans in 2012 after scoring 157 goals in 341 appearances.
His last act in a Blues shirt was to rifle home the winning penalty in the shoot-out against Bayern Munich that seized the London club's first Champions League crown two years ago.
"My desire to win is still the same and I look forward to the opportunity to help this team," Drogba said.
"I'm excited for this next chapter of my career."
Mourinho, who originally lured Drogba to Chelsea for £24million in 2004 and who said in March he expected him to return, 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 Barclays 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 amid allegations of unpaid wages, 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.