Justin Bieber either has a gigantic dong or cops are being overly cautious ... because the jailhouse video of Bieber taking a leak behind bars has an ENORMOUS black bar.
Miami Beach P.D. just released the video ... shot while Bieber was being booked at the jail.
Nine hours of video has already been released, but the judge wanted special care taken for the video showing Bieber peeing -- the judge wanted to make sure cops covered his junk before releasing it.
It's weird ... law enforcement told us they were having problems black-barring the video because Justin's penis was moving around. But the video only shows him walking to the toilet -- possibly with his penis exposed ... but the movement isn't that radical.