Oscar De La Hoya took a break from training for his return to the boxing ring ... to SING with a mariachi band on Cinco de Mayo, and TMZ Sports has the footage.
The 48-year-old hit up Maestro, a Mexican food restaurant in Pasadena (he's like the mayor over there) and started feeling himself when the mariachi band began to play.
So, Oscar -- who once released a Latin Pop album back in the day -- decided to grab the mic and take over the lead singing duties.
Look, he ain't exactly Marc Anthony .