Kate Upton -- I Had NO Liquor in My Box ... at Tigers Game

Kate Upton sucking down an ice cold beer at a baseball game might be every man's fantasy -- but the 20-year-old supermodel says she was NOT boozing illegally ... despite a suspicious video clip.

Kate was at a Detroit Tigers game on July 6th -- in Tigers ace Justin Verlander's luxury suite -- when TV cameras spotted her toasting friends with a sudsy beverage.

We all know the formula -- baseball game + luxury box + sudsy beverage = BEER!

But Kate's rep insists that's not what happened ... telling TMZ underaged Kate was not drinking beer, and was only there to take in the game with a friend. The rep would not say what Kate was drinking.

A rep for the Tigers assured us "all patrons who appear to be under 30 years old are ID'd."

No doubt, Kate looks way under 30.