No, that's not a photo of a skinny Vin Diesel ... it's "American Idol" alum Chris Daughtry!

The 32-year-old Daughtry frontman is proud of his tight stomach, posting a pic of his newly-ripped physique on Twitter last night.

"Just left the gym! After 5 yrs of neglect, I'm PUMPED to see AB's again! Hard work and commitment is paying off!"

We'd say so ... the father of four looks great!

Fellow Season 5 alum Kellie Pickler is making headlines for a transformation of her own today -- shaving her hair off to raise awareness for breast cancer. Check out those photos here.

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