Objective: To examine the underreporting of sexually transmitted infections (STI) (i.e., no STI by self-report but have at least one STI through biological testing or clinical examination) and factors associated with underreporting among female sex workers (FSWs) in China. Methods: A total of 454 FSWs were recruited from entertainment establishments in a rural county of Guangxi, China. Participants completed a self-administered survey about their demographic and working characteristics, history of STI (past or current infections), sexual history and practices, and HIV=STI-related knowledge and perceptions; 411 of the sample were also tested for syphilis, Neisseria gonorrheae, Chlamydia, Trichomonas, and genital warts. Conclusions: Underreporting of STI was prevalent in FSWs, especially among women with perceived lower STI risks. The underreporting might be largely due to their perception of low risks for STI and unawareness of STI symptoms (including asymptomatic STIs). Future studies of FSWs should identify those new in commercial sex with lower STI awareness and perceived risks and encourage them to seek timely and appropriate testing and treatment.