Alvarez (42-0-1) unified the 154-pound titles with a dominant victory, retaining his WBC super welterweight championship along with the vacant Ring Magazine belt. But it was the WBA belt that was the ultimate prize - and it wasn't his.
"This is for my brother," Alvarez said.
Alvarez's brother, Rigoberto, lost the WBA title to Trout in February 2011 in the family's hometown of Guadalajara. Alvarez returned the belt to his brother, who was in the ring when the result was announced.
Alvarez receiving winning scores of 115-112, 116-111 and 118-109 to beat Trout, from Las Cruces, N.M.
Some observers speculated the 21-year-old Alvarez wasn't ready for a veteran opponent like Trout (26-1), but they didn't realize how personal the bout was to him.
"My brother was a big motivation for this," Alvarez said. "I did this for him. He beat my brother, and that's my blood."
Alvarez fought like a man out for revenge, knocking down Trout for the first time in his career and staggering him several times to the delight of the crowd of 39,247 that continually chanted "Mexico! Mexico!"
"I'll take the loss like a man," Trout said. "The better man won tonight. He was better than me. I have no excuses."
The fighters stood toe-to-toe for much of the bout, both unwilling to tie up the other at the expense of possible power punches. Referee Laurence Cole only broke up the fighters once, doing so midway through the first round.
Trout was the more active fighter, connecting on 154 of 769 punches compared to 124 of 431, but Alvarez landed 43 percent of his power punches compared to 27 percent for Trout.
"He shocked us, I was prepared for a different fighter," Trout said. "I tried to pressure the action and change things up, but he kept changing."
Alvarez dropped Trout early in the seventh round, catching the southpaw flush to the chin with a powerful straight punch. Trout staggered a few steps backward before falling front first to the canvas.
"He caught me with a good shot," Trout said. "There is nothing else I can say."
Alvarez continued to deliver right hooks and jabs to the head, staggering Trout twice more in the round.
Both fighters kept their distance early in the opening round, with Trout delivering the first meaningful blow with a right jab to Alvarez's forehead with 1:52 remaining. Alvarez was able to press the action, stringing together several combinations to win the round.
"Austin Trout is difficult fighter, but I was smart," Alvarez said. "Little by little I figured out how to fight him. I was able to connect with my right and my jab. My jab was key."
The action picked up significantly in the second round with Alvarez delivering a series of body blows and Trout's corner nearly pulling down the ringside camera man because he was obstructing their view. Alvarez dominated the action, taking the second round, too.
Alvarez staggered Trout with a left hook to the body midway through the third round, causing the southpaw to stop and wince. Trout recovered, delivering several body blows.
Alvarez stunned Trout with a left hook to the head in the fourth round as the two fighters continued to trade blows.