The Biebs actually put the woman on blast, reposting a snippet of the so-called Selena Gomez fan begging people "to f***ing bombard" Hailey's Instagram Live comments section and bully her by saying "Selena is better" and "Go after her, please. Let's all go after her."

JB didn't let it slide, saying ... "This sad excuse of a human just encouraged people on video to literally go after my wife telling people to say that my previous relationship was better so on and so [forth]. I just wanted to share this so people get an idea of what we face on a day to day."

The singer also made a dig at the woman saying, "After watching that I could easily let it steal my joy, but then I think about her life and how miserable she must be to want to spend her life trying to make others feel small."

On the one hand, ya gotta admire Justin going all out to defend his wife, but on the other -- he did just give a troll the biggest platform she's ever had, and she's probably thrilled about it. It's a double-edged sword for the Biebers.

Hailey added her own feelings, saying ... "I would never in a million years wish for someone to be treated this way and I will never condone this kind of hateful behavior."