Background <p>Syphilis and neurosyphilis remain significant comorbidities among people living with human immunodeficiency virus (HIV). Data on cerebrospinal fluid (CSF) findings in co-infected individuals are limited. This study aimed to determine which CSF parameters and HIV-related immunologic and virologic markers are associated with neurosyphilis in people living with HIV (PLWH), and to assess the impact of antiretroviral therapy (ART) on these relationships.</p> Methods <p>We conducted a retrospective analysis of 211 people living with HIV undergoing syphilis serology and CSF testing at a tertiary care center between 2011 and 2017. Patients were stratified by syphilis stage and ART status. CSF parameters, CD4⁺/CD8⁺ cell counts, HIV viral load, and syphilis diagnostic parameters were analyzed.</p> Results <p>Neurosyphilis was diagnosed in 10% (n = 21) of the study population. <i>Treponema pallidum</i> particle agglutination (TPPA) titers ≥ 1024 in CSF were significantly associated with neurosyphilis (p = 0.001). No significant association was observed between neurosyphilis and CD4⁺ count &lt; 200/μL (p = 0.67) or detectable CSF HIV viral load (p = 0.83). ART success was associated with lower CSF viral loads (p &lt; 0.001), higher CD4⁺ counts (p &lt; 0.001), and CD4⁺/CD8⁺ ratios (p = 0.001). CSF immunoglobulin levels (IgA, IgG, IgM) differed significantly between patients on ART and those not receiving ART, whereas leukocyte counts and protein levels showed no differences.</p> Conclusions <p>High CSF TPPA titers, particularly ≥ 1024, are strong indicators of neurosyphilis among PLWH. CD4⁺ count and CSF HIV RNA were not predictive of neurosyphilis, underscoring the diagnostic challenge posed by HIV co-infection. ART success influences CSF immunological parameters but not neurosyphilis risk directly.</p>