Justin Bieber -- Styrofoam Cuppin' in Atlanta Nightclub -- Sign of Sizzurp [VIDEO]

It's better than Bigfoot or the Loch Ness monster -- it's Justin Bieber holding one of his fabled STYROFOAM cups on his way into an ATL nightclub ... a clear sign of lean ... and it's all on video.

TMZ has obtained security footage at Tongue & Groove -- showing Bieber entering the club Monday evening holding a tall, white styrofoam cup ... the telltale container for lean ... aka sizzurp.

It's the first time Bieber's been seen holding one of these cups -- and our sources say he had the cojones to continue partying with it inside.

As we reported, Bieber's drug use -- specifically lean and weed -- has spiraled out of control in Atlanta ... in fact, one source said he was constantly high ... drinking more lean, smoking more weed than ever.

We're told the singer has ordered his goons to make sure NO ONE sees him carrying the white cup ... which has resulted in multiple fights with photogs.