The objective of this study was to investigate if early syphilis infection affects markers of HIV infection; CD4 T cells and viral load (VL). A retrospective study was performed on 160 HIV-positive patients (111 receiving antiretroviral therapy [ART] and 49 without ART). Early syphilis diagnosis was made in HIV patients during their follow-up at the HIV/AIDS Unit at a Greek Dermatology and Venereology Unit. The patients’ blood tests were available at the time of diagnosis, as well as before and 12 weeks after early syphilis diagnosis. CD4 T cell counts and VL levels were measured. It was found that syphilis infection had a negative impact on the CD4 T cell counts in both groups, with reduced CD4 T cell counts observed in 84.6% (99/111) and 79.5% (39/49) of patients receiving and not receiving ART, respectively. After treatment for syphilis, CD4 T cell counts returned to pre-treatment levels in most patients, especially those receiving ART. There was a slight and transient VL increase. Patients receiving ART had a 27% increase in VL, compared to 71.4% among patients not receiving ART. Although the VL increase was slight (41–14,000 copies/ml) in the group under treatment, 4–5% (5/111) patients did not return to pre-treatment levels. Moreover, viral mutations associated with treatment resistance were identified in these patients. Early syphilis accelerates and complicates the progression of HIV infection. Early diagnosis and treatment of syphilis may prevent infection-associated complications in most instances. Consequently, prevention of syphilis and other sexually transmitted infections is of great importance for patients infected with HIV.