The Mexican dominated most parts of the round in the grueling contest to end Plant's unbeaten run at the MGM Grand Arena in Las Vegas.
Summary of the fight
The 31-year-old has now unified all the belts in the middleweight division to write his name in the history books of boxing.
Canelo dropped Plant midway to end of the of 11th round but the American got up wobbling before the count of 10 seconds.
The Mexican knocked him out again for the second time as Plant fell to ground ground faced-down as referee Russell Mora stopped the fight.
Canelo takes his record in 57 wins, one loss, two draws and 37 of them coming by the way knockouts.
His only career loss was back in 2013 to the undefeated American fighter Floyd Mayweather in a majority decision victory from the judges.
Fury takes 2 ladies who asked for selfies on a drinking spree
Meanwhile, Legit.ng also reported that WBC champion Tyson fury made the day of two beautiful ladies who asked him for a selfie while drinking at a bar, The Sun, Daily Mail.
Stella Kabelis and Kailah Downie were star-struck when they met the Gypsy King who recently retained his WBC crown last month following an 11th-round stoppage of Deontay Wilder.
The two friends went out for a drink and felt lucky when they met the 6 ft 9 heavyweight champion and his coach Andy Lee in a swanky restaurant.
Usman successful defends his welterweight crown against Covington
Legit.ng also reported that Kamaru Usman was stretched to five rounds in his rematch with Colby Covington as he retained his UFC welterweight championship.
All three judges gave the fight of the Nigerian nightmare in his successful sixth defence of his welterweight crown.
Usman had a slight edge in the first round as he connected with a couple of jabs and defended Colby's attempted take-down.