Broner dominated his opponent with speed and power in Atlantic City, sending the Mexican fighter to the floor with a good combination in round eight.
DeMarco's corner subsequently through in the towel and the referee stopped the fight.
"Once I'm in there, where I've got a gameplan and I see something, I go after it," said Broner after the fight. "I knew he would have the skills and the will to beat me."
The 23-year-old improved his record to 25 wins with 21 knockout victories.