Jeff Nippard -- a popular bodybuilder and fitness influencer -- has suffered a major loss ... his fiancée has died.

Nippard's team broke the bad news Friday, issuing a statement on Instagram about the passing of Stephanie Buttermore. Her cause of death was not released.

The statement touches on Nippard's 10-year relationship with Buttermore, who "meant the world to Jeff." It also honors Buttermore's legacy of "warmth and compassion, her love for her family, and her PhD research on ovarian cancer.” 

Her death comes as somewhat of a shock ... Nippard -- a professional bodybuilder with a massive YouTube following -- posted a pic of the two of them on Valentine's Day.

In the pic, he's sitting on a couch with Buttermore, who looks healthy as she flashes a bright smile. Nearly 2 weeks later, Buttermore celebrated her birthday on February 25.

Before her death, Buttermore had a PhD in Pathology & Cell Biology from the University of South Florida. Like her husband, she was a YouTube fitness influencer who also focused on ovarian cancer research.

Buttermore was 36.

RIP