Mexican Soto Karass, who himself was put down in the 11th by a body shot, ended proceedings with just over two minutes remaining when he landed a left hook of his own.
Referee Jon Schorle waved off the contest with Berto bloodied and beaten; the American, who had won his first 27 outings as a professional before losing to Victor Ortiz in 2011, has now lost three of his last four fights.
His hopes of getting his career back on track following a defeat to Robert Guerrero last time out were not helped by a right shoulder injury suffered in the fourth round, meaning he was left to carry on one-handed.
The 29-year-old Floridian - fighting for the first time under new trainer Virgil Hunter - was understandably out-worked by his opponent from then on.
At the top of the stoppage Soto Karass led on one judge's card, Berto on another and the third making it even.
"I thought I was winning the fight," Soto Karass - who breathed new life into his career by out-pointing Selcuk Aydin back in January - said afterwards.
"But my corner told me to go with everything. I had to go with the knockout to avoid any problems with the judges."
On the undercard, Omar Figueroa claimed the interim WBC lightweight title with a unanimous decision over Japan's Nihito Arakawa, while Keith Thurman stretched his perfect record to 23-0 - picking up the interim WBA welterweight belt in the process - with a 10th round knockout of Argentina's Diego Chavez.