Ruiz, 38, twice won the WBA heavyweight title. He won a unanimous decision over Evander Holyfield in 2001 in claiming his first world title and held that belt until March 2003. He regained the championship in a fight against Hasim Rahman late that year and kept it until April 30, 2005.
He has continued to box, losing his latest bout April 3 to David Haye by knockout. Ruiz was 44-9-1 with 30 knockouts over his career.
"I've had a great career but it's time for me to turn the page and start a new chapter of my life," Ruiz said in a statement Monday.
"I'm proud of what I 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."
Ruiz said he would return to the Boston area and open a boxing gym.