Selena Gomez Trolls Justin Bieber ... Welcome To The Internet, Rookie

Selena Gomez just owned Justin Bieber so hard ... mocking his "call to arms" against his fans.

The Biebs posted a pic of him and 17-year-old Sophia Richie over the weekend ... threatening to make his Instagram page private if 'Beliebers' continued their attack over his rumored fling.

Gomez insinuated herself in the drama, posting this gem ... "If you can't handle the hate, then stop posting pictures of your girlfriend lol -it should be special between you two only."

She ended with sage words of advice ... "Don't be mad at your fans. They love you."

Selena deleted the post -- an old trick -- knowing it would be seized and emblazoned on the Internet forever.

#MicDrop