Aldo took control of the fight from the opening horn, landing his patented hard right kicks that repeatedly found Mendes' left thigh.
Mendes was unable to take Aldo to the ground but managed to get the champion's back. That, however, eventually would lead to his demise.
Aldo was able to unhook Mendes' grasp and spin away. Once he freed himself, Aldo connected with a hard left knee that sent Mendes to the canvas.
Aldo then followed up with a punch while Mendes was defenseless on his back.
"I knew he was going to go for my legs and I knew how to throw that knee," Aldo said.
The knockout came at 4:59 of Round 1.
Immediately after successfully defending his title for the fifth time in a row, the Brazil native ran into the crowd. He was swarmed by his fans and carried back to the cage on their shoulders.
Aldo improved to 21-1.
"Jose is a great champion; he's a tough dude," Mendes said after suffering the first loss of his professional mixed martial arts career. "That was the best I'd ever felt for any fight, any camp. I was very prepared, but he caught me."
Mendes is now 11-1.
Belfort submits overweight Johnson in first
Anthony Johnson stepped into the Octagon at his heaviest ever and could not carry the weight long.
Despite getting off to a fast start, Johnson quickly became exhausted. That proved to be his downfall.
Vitor Belfort took a tired Johnson to the ground and got his back. Belfort then flattened Johnson out and applied a rear-naked choke. The submission hold was deep, and Johnson was forced to tap at 4:49 of the first round.
Johnson struggled to make weight for this fight, which originally was slated to be fought at 185 pounds. He tipped the scale Friday at 197 pounds.
He entered the Octagon weighing 211 pounds. Belfort, who weighed 186 pounds at the weigh-in, weighed more than 200 pounds Saturday night.
Although Johnson was visibly larger, Belfort was not intimidated.
"I fought big guys before; I'm not afraid of big guys," said Belfort, who improved to 21-9. "I cut 25 pounds in four days. I'm focused on my goal."
That goal is to win the UFC middleweight title. Belfort has won two fights in a row since falling to champion Anderson Silva in the first round on Feb. 5, 2011.
Johnson, who forfeited 20 percent of his purse to Belfort for failing to make weight, drops to 10-4.
Palhares gets his hooks into Massenzio
There is no secret to what welterweight Rousimar Palhares wants to do inside the cage. If he can get an opponent's heel, he will force him to tap.
Mike Massenzio did not want to get caught on the ground with Palhares. But a little more than a minute into their welterweight bout, Palhares got Massenzio off his feet and applied a heel hook.
There was no escape for Massenzio, who would tap at 1:03 of the first round.
"This is a strong point of mine," Palhares said of his heel hook submission technique. "If I am able to do it, I can do it well."
Palhares improves to 14-3. Massenzio slips to 13-6.
Referee Yamasaki disqualifies Silva
Erick Silva has a very aggressive style of fighting. He tends to jump on his opponents immediately.
And that was the style he used against Carlo Prater during their welterweight bout.
Silva used a knee to send Prater to the canvas. Once his opponent was down, Silva began landing punches.
The strikes appeared to be landing on the side of Prater's head, but referee Mario Yamasaki saw things differently.
After jumping in to stop the action 29 seconds into the bout, Yamasaki disqualified Silva for illegal strikes and Prater was awarded the victory.
"I was telling him no hitting in the back of the head," Yamasaki said. "I have to decide here right now. He hit some in the back of head."
Silva disagreed with the referee's assessment.
"I have great respect to the referee," Silva said. "But I see most of my punches hitting the side of the head. I don't see any hitting the back of the head."
Barboza's spinning heel kick knocks out Etim
But Barboza refused to allow the judges to decide his fate. He connected with right spinning heel kick to the head that knocked out Etim at 2:02 of the third round.
"In a fight, you try things and hope they work," Barboza said. "Tonight [the kick] worked. It's something I've practice a lot, and I finally was able to land it hard."
Throughout the fight, Barboza was the faster, more accurate striker. He mixed up his strikes, landing punches and kicks to Etim's head and body.
However, Etim never backed down. He was able to land some takedowns in the fight but could not keep Barboza off his feet for long periods of time.
The win allowed Barboza to extend his unblemished mixed martial arts record to 10-0.
Etim has lost two of his past three fights. He is 15-4 overall.
Tavares edges Stout by decision
Lightweight Sam Stout entered the Octagon for the first time without his trainer and brother-in-law, Shawn Tompkins, at his side. Tompkins died of a heart attack in August.
Stout was able to control his heavy heart and give Brazil native Thiago Tavares all he could handle. After a first round that saw Tavares take Stout to the ground several times, the fighters spent much of the following two rounds standing and delivering strikes.
The standup contest was close, but Stout connected regularly with straight right jabs. Those jabs would find Tavares' nose repeatedly, eventually drawing blood.
Stout also was able to stuff several takedown attempts by Tavares. But the judges favored Tavares and awarded him the unanimous decision win.
All three judges scored it 29-28 for Tavares. ESPN.com had Stout winning 29-28.
"Sam Stout was the toughest opponent I've ever faced, and this is the biggest win of my career," Tavares said.
With the win, Tavares improves to 17-4-1. He has now won two fights in a row.
Stout slips to 17-7-1. He entered the cage on a two-fight win streak.
"This was my first fight without Shawn," Stout said. "It was definitely an adjustment not having him in my corner. I hope Shawn would be proud.
"I thought I took Rounds 2 and 3, but I struggled to find my rhythm. My head wasn't in it in the first round."
Gonzaga submits Oliveira in return bout
Heavyweight Gabriel Gonzaga made his return to the UFC a successful one with a first-round submission of Octagon newcomer Ednaldo Oliveira.
After opting to slug it out in his more recent UFC appearances, Gonzaga (13-6) vowed a return to his grappling style. He ate several punches from Oliveira early but was able to get the fight on the ground.
Gonzaga soon got Oliveira's back and applied a rear-naked choke. Oliveira (13-1-1, 1 no contest) tapped at 3:22 of the first.
"I want to focus on my Brazilian jiu-jitsu," Gonzaga said. "Sometimes I don't have the time to go to the ground game, but I try my best."
After dropping three of his four previous UFC bouts, Gonzaga fought for the Reality Fighting heavyweight title in October 2011. He won that belt with a third-round submission of Parker Porter.
Early domination lifts Alcantara
After dominating the first two rounds, Yuri Alcantara did just enough to keep Michihiro Omigawa from stealing this featherweight showdown.
Alcantara landed hard left hands and nearly submitted Omigawa with an armbar in the first. He would control the second round with impressive ground skills en route to a unanimous decision victory.
The judges scored this fight 30-27, 29-28 and 30-27. ESPN.com had Alcantara winning 30-27.
Although ahead after two rounds, Alcantara (27-3) entered the third round breathing heavily. His punches lacked the pop of the previous two rounds.
Still, Omigawa (13-11-1) seemed reluctant to force the action.
Right hand, knee power Pyle to victory
A straight right hand from Pyle stunned Funch; shortly thereafter, a right knee to the chin would send Funch to the canvas.
A few more punches made the TKO official at 1:22 of the opening round.
"That's probably my best finish in UFC," said Pyle, who improved to 22-8-1. "I want to tell all the welterweights that's how I'm going to look every single time I step inside the Octagon. I'm coming to fight and I'm coming to win."
Funch drops to 8-3.
Arantes rebounds to spoil Carvalho's debut
Featherweight Felipe Arantes struggled during the first round of his featherweight bout with Antonio Carvalho but bounced back in the second and third to earn a unanimous decision.
All three judges scored the bout 29-28. ESPN.com also had it 29-28 for Arantes, a Brazil native who trains in Newark, N.J.
In the first round, Carvalho was able to get Arantes to the ground early and control him for more than 2 1/2 minutes. But Arantes (14-4) kept the fight standing in the second and was able to utilize his striking skills.
Surprisingly, a large amount of the third round was fought on the ground. But it was Arantes who did the damage there. From the top, he landed an elbow that opened a cut below Carvalho's left eye.
Carvalho, who was making his UFC debut, falls to 13-5.
Franklin McNeil covers MMA and boxing for ESPN.com. He also appears regularly on "MMA Live." Follow him on Twitter at http://twitter.com/Franklin_McNeil.