Introduction: Syphilis is a sexually transmitted infection caused by Treponema pallidum bacterium. Oral manifestations of syphilis are always a challenging diagnosis because broad clinical aspect is easily missed due to other oral manifestations of local, infectious, or systemic origins. In addition, in HIV-positive patients, primary syphilis can usually have a symptomless course. Case description:A 44-year-old HIV-positive male patient, living with the infection for 17 years, working as a sex worker, presenting primary syphilis with syphilitic ulceration of the tongue. The lesion was unique, indurated, with irregular margins and whitish/red base. The patient reported no pain or discomfort in the lesion. This last information along with a detailed oral clinical exami nation was important to help exclude initial diagnostic hypothesis of traumatic ulcer and other oral manifestations. Diagnosis of syphilis was only made after clinical history of patient being associated with data of chemiluminescent microparticle immunoassay and venereal disease research laboratory.Conclusions: Health professionals should be aware of possible oral manifestations of primary syphilis, especially at risk groups. Moreover, the association of clinical history with laboratory tests is necessary for final diagnosis.