Los Angeles Chargers rookie Justin Herbert just chopped it ALL off on top ... with the QB debuting a brand-new look on Wednesday.

The 22-year-old signal-caller has made a name for himself since his Oregon days for his skills on the field ... but was just as attention-grabbing when the helmet came off.

Matter of fact, the guy had such great flow ... he got an endorsement deal from Head and Shoulders!!

But, that is all a thing of the past ... with Herbert straight-up buzzing it all off.

So ... why?!

"No particular reason," Herbert told reporters. "It was just time and it was getting too long, so it was time to cut it."

JH added it had nothing to do with the Chargers' 3-game losing streak ... saying he's not a superstitious guy.

"Honestly, it was just getting long and time to cut it."