The 36-year-old Rivera was second best from the outset and was eventually pulled out by the ringside doctor due to a cut over his left eye.
Purdy took his record to 20-3-1 and has won his last four fights by stoppage after losing his British welterweight title over 12 rounds to Colin Lynes.
Rivera took the fight at just six days' notice after Carson Jones pulled out with an illness and put up a brave effort - although he never looked remotely likely to cause an upset.
Purdy put his opponent on the canvas just a minute into the fight with a right hand to the temple and although Rivera was hardly shaken, it set the tone for a one-sided contest.
He had too much power and strength for Rivera and was able to completely dominate.
A right hand cut Rivera in the seventh and although he responded by finally rattling Purdy in the eighth and driving him back across the ring, Purdy shrugged off the setback.
More solid punches in the ninth caused the cut to worsen, and although a more competitive fight may have been allowed to continue, the ringside doctor made the right decision to spare Rivera further punishment.
Purdy, who had failed to make the weight to fight for the IBF International title, told Sky Sports: "I did what I had to do, I'm a fighter at the end of the day and I would have stayed in there right to the end.
"A few things went wrong coming up to the fight with Carson Jones pulling out. I took my foot off the gas for a couple of days and struggled to make the weight a little bit, but it is the biggest lesson I have ever learned in boxing and this will make me a stronger fighter."