Fortuna had been stripped of his belt after failing to make the 126-pound weight limit a day earlier.
''I was surprised when I didn't make the weight,'' Fortuna said after stopping Zamudio in the first round. ''Once that happened, I wanted to get a knockout to impress everyone.''
The fighter from the Dominican Republic arrived at the weigh-in at 127 pounds before dropping to 126.8 after a 30-minute workout. Not even stripping out of his clothes or standing on one leg helped him get under the limit.
The result was a determined Fortuna, who attacked Mexican Zamudio from the outset and dropped him in the opening seconds of the bout.
Zamudio managed to regain his feet but with just 1:08 on the clock, he was left unconscious on the canvas by a flurry of blows.
''I was surprised he got up from the first knockdown because I hit him so hard,'' Fortuna said. ''I knew he wasn't getting up the second time because he fell pretty awkwardly.''
Victory for Fortuna took him to 22-0 (15 KOs) while Zamudio, who left the ring on a stretcher and in a neck brace bound for hospital for testing, dropped to 25-2-1.
''I'd say he was unconscious for two or three minutes, but we gave him some oxygen and that seemed to help,'' ringside physician Eric Wurmser said. ''That was a pretty incredible knockout.''