Up-and-coming rapper Luci4 -- best known for his viral hit "BodyPartz" -- died earlier this year ... and now we know the cause.

According to the Los Angeles County Medical Examiner ... the musician -- born James Dear -- died of an accidental fentanyl overdose.

We broke the news ... the 23-year-old was found dead on February 22, at a friend’s house in L.A.

The Los Angeles Fire Department told us at the time ... they responded to a medical call at the house at 11:40 AM ... but when they arrived, the person was already deceased, so police were notified.

Luci4 made waves online as a pioneer of the internet microgenre Sigilkore ... blowing up on TikTok in 2021 and eventually landing a deal with Atlantic Records.

"BodyPartz'' exploded across social media and went gold, certified by the Recording Industry Association of America.