Johnny Manziel -- I Can Booze With My Mama in Texas

Johnny Football could've chugged as much Dom Perignon as he wanted to on Saturday night and it wouldn't have mattered to police ... 'cause his MOTHER was right there next to him ... TMZ has learned.

As we previously reported, 20-year-old college football phenom Johnny Manziel hit up Avenu Lounge nightclub on both Friday and Saturday night after leading Texas A&M to a victory in the Cotton Bowl.

During the festivities, Johnny posed for several pics while clutching a bottle of Dom ... which raised several red flags, considering he's under 21.

But in Texas, there's a HUUGE loophole to the drinking law -- it's OK for someone under 21 to booze in a bar as long as that person is with a parent who consents.

We spoke to Shaul Sultan -- the owner of Avenu Lounge -- who confirms Johnny's mother WAS with him on both nights ... so the Heisman winner couldn't have broken any drinking laws. (UPDATE: We're told Johnny's father was there too).

Still, the owner says nobody working in the lounge that night ever saw Johnny consume alcohol either night.

In fact, the owner says the bottle of Dom belonged to another customer who had a table by the DJ booth ... and Johnny only used the bottle as a prop for some goofy photos.

The owner says Johnny never opened a tab at the bar.

FYI -- the parent loophole isn't just in Texas ... it's also in play in Connecticut, Wisconsin, Wyoming, Massachusetts, Mississippi, Nevada, Louisiana, New York and Ohio.