Syphilis has been controlled since 1960. However, syphilis is now reappearing in some areas in Africa and South-east Asia including Thailand. In Thailand, syphilis is reappearing in all regions. The most high-risk population is the youth aged 15-24 year. Objective of this study aimed to determine factors associated with knowledge and awareness of syphilis prevention among youths visiting at family planning clinic, Bangkok, Thailand. Method of this study was a cross-sectional study conducted by purposive sampling among the youth aged 15-24 years old who visit at PPAT family planning clinic, Bangkok, Thailand during April-June 2021. Multivariable logistic regression analysis was used to determine the association between factors and the knowledge/awareness level of syphilis prevention. Total 207 participants were recruited in this study. The result from multivariable logistic regression analysis showed that unemployment associated with lower knowledge level compared with fulltime job [OR 0.29, 95% CI (0.11-0.75)], living with family or relative associated with lower knowledge level compared with living alone [OR 0.187, 95% CI (0.07—0.51)], always use of condom associate with higher knowledge level compared with never use condom [OR 7.95, 95% CI (1.64-38.61)], participant who never had a reproductive health education associated with lower knowledge level compared with participant who had a reproductive health education [OR 0.29, 95% CI (0.11-0.77)], and participant who had ever used a reproductive health service at private clinic/hospital associated with lower knowledge level compared with who use this service for the first time [OR 0.27, 95% CI (0.07-0.99)]. According to the awareness, participant who had a good knowledge level associated with higher awareness level of syphilis prevention compared with those who had fair or poor knowledge level [OR 1.19, 95% CI (1.06-1.33)]. We concluded that the youths were recognized as vulnerable group for syphilis. To prevent them, it was important for public health to emphasize on knowledge and awareness regarding syphilis prevention among the youths. Additionally, novel strategy to increased knowledge and awareness level of syphilis prevention need to be considered.