Justin Bieber -- Hey Selena Gomez ... Beer Me

So much for Selena Gomez being a good influence on Justin Bieber ... TMZ has obtained a photo of the underage singer boozing at a 4th of July party in L.A. -- and Selena just stood by and watched.

The photo was snapped at a Hollywood Hills house -- where the drinking age is still 21 -- and as you can see, Bieber is cradling a beer bottle (Dos Equis to be exact) ... squishing his crotch against Selena.

As we first reported, Selena gave Bieber another shot ... reuniting with him on the condition he clean up his act, and cut the antics -- pot smoking, speeding and bucket peeing -- with his buddies.

Apparently, 20-year-old Selena -- like almost every single college-aged kid -- is still cool with underage drinking though.

As for his taste in brew -- Justin Bieber doesn't always drink beer, but when he does ... he prefers to wear shorts made out of salvaged traffic cones.

Stay thirsty my friends.