MTV's "Ridiculousness" star Sterling "Steelo" Brim is mourning the loss of his 3-year-old nephew who drowned in his swimming pool ... TMZ has learned. 

Law enforcement sources tell TMZ ... the boy wandered into the North Hollywood backyard Friday morning when adults weren't looking and fell in the pool. It's unclear how long the boy was in the pool before he was discovered.

We're told paramedics rushed to the house and the child was taken to a nearby hospital. He was pronounced dead just after noon the same day.

We're told the boy's family was visiting Steelo from Chicago. 

The death's been ruled an accidental drowning.

Steelo tells us, "Last weekend, my beloved nephew passed away in a tragic accident. We will miss him dearly and ask for prayers and privacy during this time of mourning."