The 38-year-old American, nicknamed 'The Quiet Man', will bow out with a pro record of 44 wins – 30 coming by knockout – nine losses, one draw and one no contest and having become the first heavyweight world champion of Hispanic descent.
His final defeat came in a ninth-round stoppage in Manchester as Britain's Haye defended his WBA title for the first time on 3 April.
Haye sent Ruiz to the canvas four times in the first six rounds and the American took the defeat as a signal to call it quits.
"I've had a great career but it's time for me to turn the page and start a new chapter of my life," said Ruiz. "It's sad that my final fight didn't work out the way I wanted, but, hey, that's boxing.
"I'm proud of what I've accomplished with two world titles, 12 championship fights, and being the first Latino heavyweight champion of the world.
"I fought anybody who got in the ring with me and never ducked anyone. Now, I'm looking forward to spending more time with my family."
An awkward opponent with a reputation for jabbing and holding, Ruiz won his first heavyweight crown in 2001 when he won a unanimous decision over Evander Holyfield for the WBA title.
Ruiz said he now plans to move from Las Vegas to his hometown of Boston in order to open a boxing gym in the inner city.
He added: "With my experiences in boxing, I want to go home and open a gym where kids will have a place to go, keeping them off of the streets, so they can learn how to box and build character.
"Someday, I'd like to see one of them go on to represent the United States in the Olympics. I want them to have the same opportunity to see the world that I had as an amateur."