Background/Objectives: Onychomycosis is a fungal nail infection that may present with severe, atypical, or treatment-resistant features in people living with HIV. Despite its clinical importance, evidence regarding its epidemiology, causative agents, and relationship with immune status remains limited. This systematic review aimed to evaluate the association between onychomycosis and HIV, focusing on prevalence, clinical characteristics, etiologic agents, and CD4+ T lymphocyte counts at diagnosis. Methods: A systematic review was conducted following PRISMA guidelines. MEDLINE/PubMed, SciELO, Scopus, and Scilit were searched for studies published between October 2015 and July 2025 in English and Spanish. Eligible studies included case reports, case series, and observational studies involving people with HIV and confirmed onychomycosis. Data extraction was performed independently, and findings were analyzed descriptively. Results: Thirty studies comprising 1296 patients were included; 306 had detailed clinical descriptions. Most cases were reported in the Americas (85.8%) and predominantly involved male patients. CD4+ counts were available in 123 individuals; 52% had <200 cells/µL, including 18 with <50 cells/µL. Trichophyton rubrum was the most frequently identified etiologic agent. Conclusions: Onychomycosis in HIV shows etiologic diversity and commonly affects patients with advanced immunosuppression, though it may also occur with partial immune preservation. Prospective standardized studies are needed.