Peyton Manning -- PIZZA GOD ... NFL Star's 21 Pizza Joints Are BOOMING

It pays to get into business with Peyton Manning ... TMZ has learned, the 21 Papa John's franchises he bought last year are KILLING IT right now -- and employees are calling it the "Peyton Factor."

We called up each of Peyton's 21 Papa John's in the Denver area, and the story is the same everywhere: business has greatly improved since the Broncos QB took the helm.

Before Peyton purchased the locations, several restaurants reported sales of $3,000-$4,000 on any given game day -- and that number has jumped up to roughly $5,000-$6,000/day ... which equals about 60 pizzas an hour.

One location reported a 25% increase in total sales since last year.

It helps that Peyton's killing it on the field too -- his Papa John's locations offer 50% off on Mondays after a Broncos win, and so far they're undefeated. Fans recognize Peyton's stores because they all feature cutouts of PM and Papa John ... and he even makes personal visits from time to time.

To compare, neighboring Papa John's locations NOT owned by Peyton reported no significant improvement in sales.