Todd Marinovich -- My Art Business is BOOMING!!

Former Oakland Raiders quarterback Todd Marinovich has finally found success off the field ... as an artist ... and dude tells TMZ he's been blowin' up ever since his work was featured on an ESPN documentary this weekend.

Todd -- who battled serious drug addiction during and after he fizzled out of the NFL back in 1993 -- has been painting and sculpting non-stop ... telling us, "I've been sober for over three years ... the art is my main therapy."

Some of Todd's work was featured in the new ESPN documentary, "The Marinovich Project" ... and T.M. says the exposure is having an insane impact on his business, "Orders are pouring in for the prints."

Todd tells us the main piece of art featured in the doc is called, "The Alchemist" ... which is based on himself. "It's the most sought after print," Todd explains ... "I'm only making 100 of them and I'm almost sold out."