Luke Bryan -- DRINKING WHILE DRIVING ... And Cops Say It's Legal

Country music superstar Luke Bryan -- the guy behind the hit "That's My Kind Of Night" -- slammed a beer WHILE driving his truck back in July ... but cops say he did NOTHING wrong.

The incident went down on Luke's private ranch in Tennessee -- Luke can be seen chugging a can of Busch ... the brew that's "as smooth as it's name."

But according to the Knoxville Police Department, Luke was within his legal right -- 'cause in TN, it's perfectly OK to booze while operating a motor vehicle AS LONG as it's on private property.

So, while it might not be the safest thing to do -- it definitely ain't illegal.

Still, kids -- don't drink and drive.