The exact circumstances surrounding her death are not clear right now, but Maryanne's body was found Monday morning ... according to multiple reports. No foul play is suspected.

The elder Trump was a longtime attorney, and even served as a federal judge in New Jersey before she retired in 2019.

She was reportedly a close confidant of Donald's back in the day, and some have suggested he once took her advice and counsel very seriously -- that is, until they had an apparent falling out once he was elected President in 2016.

This all came to light in 2020 when Don and Maryanne's niece, Mary Trump, started trashing her famous uncle ... not to mention releasing tapes that appeared to show Maryanne also dogging her younger bro.

In some of those recordings, she could be heard accusing Donald of "lying" and having "no principles." Maryanne was one of Donald's 2 last living siblings -- his brothers Fred and Robert had already died.

Now, he just has his other older sister, Elizabeth, who's still alive.

Maryanne was 86.