Andy Ruiz turned 30 years old on Wednesday -- and he celebrated by hanging out with inmates at one of the most infamous prisons in Mexico.
Just days before the heavyweight boxing champ kicks off training camp for the Anthony Joshua rematch, he visited Topo Chico prison in Monterrey, Mexico ... where he was cheered by the hundreds of prisoners like a conquering hero!!!
It just so happened, the prison was hosting the prison championship boxing tournament that day -- so Ruiz got to see some of the inmates flex some real athletic ability.
During the visit, Ruiz spoke with the inmates and encouraged them to follow their dreams and get their lives in order ... noting that he was on a bad path himself before turning his life around and becoming a boxing superstar.
Andy's rep tells us, "For his birthday he decided to spend the day with these guys and talk to them and motivate."
Ruiz will get a second crack at Joshua on Dec. 7th in Saudi Arabia.
By the way, Topo Chico prison is notorious for a 2016 prison riot in which 49 people were killed. Two rival factions of the infamous Zetas cartel went to war inside the facility ... setting the cell blocks on fire.
In 2011, police found the body of a prison official -- completely dismembered -- inside of a box in an abandoned car near the penitentiary.
There's more ... but it's similar stuff.
Point is ... it ain't exactly the safest place in the world -- but Ruiz is beloved in Mexico, he was never in any real danger.