Santa Cruz extends his perfect record to 28-0-1 16KOs while fellow Mexican Manuel Roman dropped to 17-3-3 6KOs.
The referee stops the contest 55 seconds after the second round.
Mayweather is guaranteed $32 million for stepping into the ring with Maidana set to earn $3 million.
Mayweather's career earnings top $420 million with the Maidana fight, according to forbes.com.
The second world title fight on the Las Vegas undercard sees Leo Santa Cruz (27-0-1 15KOs) defending his WBC super-bantamweight title against fellow Mexican Manuel Roman (17-2-3 6KOs).
Mayweather and Maidana have disappeared to their dressing rooms to begin their preparations.
Two judges scored 115-113 for Vazquez and Bey while one scored 119-109 to the new champion.
Bey extended his record to 21-1-1 10KOs while Vazquez dropped to 34-4 13KOs.
American Mickey Bey is the new IBF lightweight champion winning a split decision against Miguel Vazquez of Mexico on the undercard of the Mayweather vs Maidana Mayhem at Las Vegas.
Are you ready for the #Mayhem that unbeaten American Floyd Mayweather Jr and Argentine slugger Marcos Maidana are set to unleash at MGM Grand Garden Arena in Saturday night in Las Vegas in just over an hour's time.
Mayweather (46-0, 26 KOs) and challenger Maidana (35-4, 31 KOs) will square off for the World Boxing Association welterweight title and World Boxing Council welterweight and junior middleweight crowns.
The Las Vegas showdown is a rematch of their world title fight on May which Mayweather won by a majority decision.
Mayweather, 37, almost never gives rematches, but he said he couldn't wait to get another crack at Maidana, who outpunched the American in the first fight.
This is just the second rematch Mayweather has given since turning pro in 1996.
Maidana, 31, came the closest of anyone to becoming the first fighter to beat Mayweather.
His aggressive game plan worked for the first six rounds, especially when he got Mayweather on the ropes. He landed a total of 221 punches on Mayweather, the most of any opponent.
The only other fighter to reach 200 punches on the champion was Jose Castillo, with 203 in 2002. Castillo is also the only other boxer Mayweather has given a rematch to.
PREVIEW
Known for his stingy defence, boxer Floyd Mayweather is looking more vulnerable than any other time in his career ahead of Saturday's world title rematch with Marcos Maidana.
The 37-year old undefeated champ has been taking more punches lately in the ring, as Argentine slugger Maidana came the closest of anyone to beating Mayweather in their first fight in May.
Maidana landed 221 punches in that fight against Mayweather, who hasn't lost since the semi-finals of the 1996 Olympics.
"I am going to try to change history and change Mayweather's record," Maidana said at Friday's weigh-in at the MGM Hotel and Casino. "I am going to come out aggressive like I came out in the first fight."
The atmosphere inside the MGM's Grand Garden Arena was electric as the two fighters made their way on to a raised stage for Friday's televised weigh-in.
Mayweather (46-0) weighed in a half pound less than Maidana for Saturday's welterweight world title fight.
Maidana was the first to enter the arena, stripping off his light blue and white track suit and stepping on the scales wearing white boxers with blue stripes. He tipped the scales at 146 pounds (66 kilograms).
Mayweather looked fit and trim in his green track suit and ball cap which read "Money just make it". He tipped the scale at 146.5 pounds.
"I had a tremendous training camp," Mayweather told the crowd. "I am ready to fight. I know I can make adjustments."
Like their first bout four months ago, the rematch will be fought at 147 pounds.
After weighing in at 146.5 pounds for their first fight, Maidana spent the next day hydrating and gained over 15 pounds. Mayweather stepped in the ring at 146.
Maidana's trainer Robert Garcia said he wants his fighter to come in much lighter on Saturday, around 150 pounds.
In the first fight, Maidana (35-4) hammered Mayweather along the ropes before the Argentine slugger ran out of steam in the later rounds.
"We can do better. We got to pressure him all the time," trainer Garcia said Friday.
Mayweather's longevity in the sport has been attributed to brilliant counterpunching skills and a solid defence that doesn't allow opponents to land many clean shots.
Mayweather, who is the highest grossing athlete in the world, has also survived so long by taking huge chunks of time off. Between December 2007 and May 2013 he fought just four times, going 40 rounds.
Mayweather said he's ready for Maidana to come out aggressively and said it is also important he keeps his composure for all 12 rounds.
"I don't worry about the pace. I worry about being me, being smart and winning the fight," said Mayweather, who is trained by his father. "I have to take my time, not rush and listen to my dad's instructions."
The undercard features American Leo Santa Cruz defending his WBC super bantamweight title against Mexico's Manuel Roman, and Miguel Vazquez, of Mexico, battling Mickey Bey, of the US, for the IBF lightweight world title.