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.