A Perth woman has been charged with the sexual assault of a nine-year-old boy.

Child abuse detectives have charged the 27-year-old, from a suburb in the city's south, with one count of sexual penetration of a child under 13, and one count of threats to injure, endanger or harm a person.

Police alleged the assault occurred between October and November last year, and said the boy was known to her.

The woman is due to appear in the Mandurah Magistrates Court today.