Brian Austin Green is once again getting in the middle of Machine Gun Kelly and Megan Fox's drama ... but making it clear he's firmly on his ex's side.

After MGK tried to downplay reports that things were icy between himself and baby mama Megan, the "Beverly Hills, 90210" alum posted to his Instagram Stories ... and urged the rapper to stop worrying about the peanut gallery.

Green wrote ... "Bro. Just be honest for once in your life. Stop caring so much about how you’re perceived that you will try and drag other people.”

Brian, who shares kids Noah, 12, Bodhi, 10, and Journey, 8, with Megan, has made it no secret he is not MGK's biggest fan.

TMZ broke the story in December ... Brian slammed MGK amid his breakup with Megan and noted the musician needed to "grow up," especially during MF's pregnancy.

Megan and MGK confirmed they had a little one on the way back in November ... however, the formerly engaged couple went their separate ways the next month.

Sources told TMZ at the time the split came after Megan found upsetting material on MGK's phone ... causing the singer to bail early on their Vail, Colorado trip.

Fast-forward to January ... when sources told us Megan and MGK weren't even SPEAKING to one another, even as the "Transformers" actress' March due date looms.

Looks like MGK's song "Lonely Road" has a whole new meaning these days!!!