The fight will be live and exclusive to DAZN on PPV in the US, Canada and around the world, including the UK.
Alvarez has decided to fight Golovkin - as originally scheduled prior to his shock defeat against Dmitry Bivol in May - and will defend all four of his super middleweight titles.
Mexico's Canelo and Golovkin have shared 24 exhilarating rounds. The first one was a split draw in 2017 before Canelo claimed a win in their 2018 rematch via majority decision.
Both bouts took place in Las Vegas and a venue for the trilogy encounter is yet to be confirmed although the T-Mobile Arena is a front-runner with the MGM Grand already booked out for that date.
"I feel very happy and proud to be able to give the best fights and this fight won't be an exception," said Canelo.
Golovkin meanwhile kept it short and sweet. The 40-year-old said: "I hope to see you on September 17."
Promoter Lou DiBella told Planet Sport this week that 'now is the right time' for the pair to end their rivalry once and for all.
"I don't mind the Canelo fight with GGG - the last two fights certainly warranted a third fight though I really favour Canelo strongly in the third fight.
"I have a hard time seeing GGG winning but he looked pretty damn good in the second half of his last fight.
"Canelo did not have a great performance against Bivol so if the fight is ever going to happen then now is the right time."
What next for Bivol?
The Russian is not short of options but he could wait and face the winner of Artur Beterbiev and Joe Smith Jr. It would determine the undisputed champion at light heavyweight.
Should Bivol not face either fighter then Joshua Buatsi is a possible option and according to Eddie Hearn, the British star is ready to challenge for world honours although his trainer may disagree.
After his win over Craig Richards, Hearn suggested he could face Bivol and win only for Virgil Hunter to say "I can't honestly say that right now because being at the world title level there are just some things that I need to see, so I can't honestly say that.
"But he (Buatsi) will always be in the fight. Trust me, he'll always be there. I'm not saying that he would lose to Bivol or Canelo.
"I think he give Canelo fits [and], on top of his physicality, would give Canelo fits and he would wear Caenlo down," Hunter said.