Adult-film actress Sophia Leone died from a fatal overdose … TMZ has learned.

Police in Albuquerque, NM tell us the late porn star’s official cause and manner of death is an accidental OD … though it’s unclear what specific substances were involved.

As we first reported … cops in New Mexico considered Sophia’s death “suspicious” and “unique” after she was found dead March 1 in her home.

During the death investigation — which at one point included a homicide detective — Sophia’s mother told police her daughter drank heavily at times and was known to have suicidal thoughts.

Still, cops say Sophia’s body showed no signs of trauma … as they’ve now officially classified her fatal OD as accidental.