Background: Poor knowledge about syphilis might contribute to its dissemination. This study aimed to assess the knowledge of the urban population of Rio Grande do Sul about syphilis and its associated factors, including presence of active syphilis infection. Methods: This cross-sectional study was representative of the urban population of the state of Rio Grande do Sul. Data collection, consisting of an in-person interview and capillary blood collection, was carried out between December 2020 and December 2022. Laboratory tests and quality control procedures were concluded by February 2023. Knowledge about syphilis was assessed by means of six items in the questionnaire: A)"Can a person that looks healthy be infected by syphilis?"; B)"Is there a cure for syphilis?"; C)"Is using a condom during sexual intercourse the best way to avoid syphilis?"; D)"Can syphilis infection increase the risk of transmitting or acquiring HIV/AIDS?"; E)"Can a pregnant woman infected with syphilis transmit the disease to her baby?"; F)"Can syphilis be transmitted by a handshake?". Participants that provided at least 4 correct answers were considered having adequate knowledge. Active syphilis infection was determined by a positive enzyme-linked immunosorbent assay and a positive confirmatory test (Venereal Disease Research Laboratory test or Treponema pallidum Hemagglutination Assay). Comparisons between those with adequate versus inadequate knowledge were made using chi-squared tests analyzing sex, age, educational level, condom use in the last sexual intercourse and active syphilis infection. Analyses took sample weights into account. Results: In total 7963 participants contributed with valid data to the study. Mean age was 49.1 (SE ± 0.7) years and 64.2% were female. Overall, 85% of participants presented adequate knowledge about syphilis. Separately, proportions of correct answers for A to F were: 89.3%; 61.1%; 93.6%; 69.8%; 68.1% and 85.6%, respectively. Age, educational level, condom use and active syphilis infection were associated with knowledge about syphilis. Conclusion: In this population of urban adults from Rio Grande do Sul, being younger and more educated was associated with adequate knowledge about syphilis. Active syphilis infection was also associated with adequate knowledge. [ABSTRACT FROM AUTHOR]