Gabriel Ruelas, tuning up for a title shot in the near future, was too strong and too quick for Paris Alexander, who was sent to the canvas four times over six-plus rounds.
After knocking Alexander down three times before the seventh, Ruelas opened the round with a left that caught Alexander on the chin. Alexander, who was knocked down twice in the sixth, collapsed backward to the canvas, and his corner immediately threw in the towel.
The official time of the technical knockout was 14 seconds of the seventh before 2,214 at the Olympic Auditorium.
"I didn't think I was hitting him that hard," Ruelas said. "Maybe I was, but he was just moving so much. I've never fought anybody who was running like that."
Ruelas (38-2, 21 knockouts), the No. 1 contender in the World Boxing Council junior-lightweight rankings, is due for a mandatory challenge against new champion James Leija within 90 days.
The only difficult moment for Ruelas, who was knocked down by Raul Hernandez in his last bout, was when Alexander caught him with a quick right to the forehead in the final seconds of the first round, wobbling Ruelas against the lower ropes as the bell sounded.
Ruelas answered in the second and third, controlling Alexander (16-9-3), who was dominated by Oscar De La Hoya last year. Ruelas also scored a knockdown in the fourth.
In the 10-round co-feature, Long Beach heavyweight Jeremy Williams, who suffered his first professional loss last March to Larry Donald, knocked out Andrew Stokes at 39 seconds of the first round.
Williams (16-1, 12 KOs) and Stokes exchanged light punches in the middle of the ring for a few seconds after the opening bell, then Williams leaned in and connected with a short right to the side of Stokes' head that dropped Stokes (14-12-1) head-first to the floor, where he stayed for the 10 count.
"Joe (Goossen, Williams' new trainer) told me to take my time and show the public that the Larry Donald stuff was not me," Williams said.