0224 GMT February 21, 2019
Alvarez showed skill and durability to outlast Golovkin in another attritional contest between the two evenly-matched rivals at the T-Mobile Arena, one year after they had fought to a controversial draw at the same venue, AFP reported.
Alvarez inflicted the first defeat of Kazakh slugger Golovkin's career after two judges scored it 115-113 in his favor, with another judge scoring it even at 114-114.
"I am a great fighter, and I showed it tonight," said Alvarez, roared on by a partisan crowd throughout on the eve of Mexico's Independence Day celebrations.
"If the people want another round, I'll do it again," added the 28-year-old, who improves to 50-1-2. "I feel satisfied because I gave a great fight. It was a clear victory."
Golovkin, who fell to 38-1-1 after the loss, left the ring immediately to get eight stitches to a cut above his right eye.
He later said he believed he had done enough to win – and opened the door for a possible third instalment.
"I'm not going to say who won tonight, because the victory belongs to Canelo according to the judges," the deposed WBA, WBC and IBO middleweight king said.
"I thought it was a very good fight for the fans, and very exciting. I thought I fought better than he did."
When asked about a third fight, he said, "Under the right conditions, yes."
Alvarez's victory ended the 36-year-old's dream of securing a record-breaking 21st consecutive defense of his middleweight titles.
While Golovkin appeared to take issue with the result, there was little sense of controversy about Saturday's outcome.
Golovkin's trainer Abel Sanchez was among those who had few complaints.
"I had it close going into the 12th round," Sanchez said. "We had good judges who saw it from different angles - I can't complain about the decision, but it's close enough to warrant a third fight.
"Canelo fought a great fight, congratulations."
Both fighters fought cagily from the outset, reluctant to trade heavy blows.