Mikayla Matthews is done with "The Secret Lives of Mormon Wives" ... saying no paycheck is worth crossing her moral line.

The Hulu star announced Monday she's leaving the reality show ... saying she recently learned some news that made her realize she could no longer keep filming.

Mikayla didn't spell out exactly what pushed her over the edge, but she mentions her own experience with "multiple forms of abuse." She says continuing to profit from certain behavior feels "wrong and unethical" and like a disservice to the children involved.

She says staying would mean going against her morals ... and no paycheck is worth that.

As TMZ previously reported, Season 5 production was thrown into chaos back in March after a heated blowup between Taylor Frankie Paul and Dakota Mortensen forced cameras down. Production later resumed, but Taylor's participation was limited.

The series has already been renewed for Season 6, with 20 new episodes ordered ... though casting details have largely been kept under wraps.

Mikayla and Taylor have also publicly butted heads in recent months, and fans are speculating her mystery "news" may involve Taylor's future with the show -- though Mikayla did not say that herself.

For now, Mikayla's drawing the line ... and walking away.