Justin Bieber’s bodyguard knocked out an overly agressive photograher with one serious punch at LAX Monday afternoon … and the whole melee was caught on tape.

TMZ obtained video of the altercation between the Bieb’s protector and the paparazzo, when the clip begins the bodyguard already has the pap in a headlock.

After several moments of grappling, the photog breaks free to square off with the bodyguard — BIG mistake … ’cause that’s when the bodyguard uncorks a straight right to the photog’s face.

As TMZ first reported, according to Bieber’s camp the altercation began when the pap jumped in front of Bieb’s SUV and refused to stop snapping pics.

The pap was arrested and booked for assault with a deadly weapon … a felony.