TechInAsia claims that the iOS service returns listings for escort services, nightclubs and other entertainment venues when told: "I want to visit prostitutes."
Siri was also found to return listings for adult websites when pornographic content is requested, despite such content being outlawed in China and Apple's assurances that the voice assistant is family-friendly.
The voice technology, which launched with the iPhone 4S in 2011, has previously made the headlines over claims that it is anti-abortion.
Bloggers and rights groups complained that Siri failed to flag up nearby abortion clinics, and in some cases it suggested pregnancy advice centres instead. Apple has since denied such allegations.
The Chinese authorities are known to impose heavy penalties on users found guilty of pornography offences, but it remains to be seen whether they will be taking action against Apple.