Drogba left Chelsea on a dramatic high in 2012, scoring the winning penalty in the shoot-out which clinched the UEFA Champions League title against Bayern Munich, the crowning moment of an eight-year stay with the Blues.
The 36-year-old joined Shanghai and then Galatasaray, but once his contract with the Turkish side ended and he became a free agent he was linked with a return to Stamford Bridge and Italian champions Juventus.
Chelsea boss Jose Mourinho - who first signed Drogba for the Blues from Marseille in a £24m deal in 2004 - insists sentimentality had nothing to do with his decision to bring the Ivory Coast striker back to West London.
Mourinho told the club’s website: “He’s coming because he’s one of the best strikers in Europe.
"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.”
Drogba said: "It was an easy decision - I couldn't turn down the opportunity to work with Jose again.
"Everyone knows the special relationship I have with this club and it has always felt like home to me.
“My desire to win is still the same and I look forward to the opportunity to help this team. I am excited for this next chapter of my career.”
Drogba was named Chelsea's greatest ever player in a poll of fans in 2012. Only Frank Lampard (211), Bobby Tambling (202) and Kerry Dixon (192) have scored more goals for the club.
He scored 157 goals in 341 games in his first spell, winning three Premier League titles, four FA Cups, two League Cups as well as the Champions League triumph.
He was named Chelsea's Player of the Year and Players' Player of the Year and has won the Premier League Golden Boot twice.
Drogba scored nine goals in nine cup finals for the Blues and his 34 goals in European competitions is a Chelsea record.
He played against Chelsea for Galatasaray in the Champions League last season, and was given a rousing reception by Blues supporters.
An Ivory Coast great, he played in all three of his country's matches at the World Cup in Brazil, leaving him with 104 caps and 64 international goals to his name.