The “Hayemaker”, making his first title defence, came out blazing at the MEN Arena and had the 38-year-old mandatory challenger on the canvas twice in the opening round.
Ruiz, who has held the belt twice, went down on his knees twice more in the following rounds before his corner threw in the towel for Panamanian referee Guillermo Perez to stop the contest after two minutes and a second of the ninth.
With 20,000 fans roaring their approval in the first heavyweight title fight in Britain for 10 years, Haye looked a genuine champion in a division crying out for such a crowd-pleaser.
“I knew it was going to be tough, I knew when I did land my bombs I would hurt him, but he’s not the type of guy to get knocked spark out,” gasped Haye of a bloodied but resilient opponent.
“I knew it would go something like that, I didn’t anticipate getting hit as much as I did in that fight but credit to John Ruiz.
“I believe I am the most exciting heavyweight in the world and even against John Ruiz I can make it exciting,” said the 29-year-old after winning only his fourth fight as a heavyweight.
The defeat ended Ruiz’s dream of joining Muhammad Ali and Evander Holyfield as the only men to become three-times world heavyweight champions.
Former cruiserweight champion Haye won the title from Russian giant Nikolai Valuev in November.
