Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) infection is associated with myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T cell leukemia/lymphoma (ATLL), which is a malignancy of mature T lymphocytes. HTLV-2 pathogenesis for humans remains undefined. As they share the same transmission routes, co-infection with human immunodeficiency virus (HIV) and HTLV-1 are often reported among the world, mainly among patients living in highly endemic areas like South America and sub-Saharan Africa. Nevertheless, many clinicians are not aware of the potential risks of co-infection with HTLV-1 when treating an HIV patient. Since both viruses infect CD4+ T lymphocytes, scientists have investigated interactions at the cellular and molecular levels, clinical associations, and related complications. Studies have shown that co-infection with HTLV-1 resulted in an increased CD4+ T lymphocyte count, which might be mistaken for immune compatibility, and lead to a delay in the establishment of antiretroviral therapy (ART) in HIV-positive patients. Some authors have observed that co-infection with HTLV-1/HIV-1 can lead to an acceleration of AIDS progression and lessening survival times. Even so, conflicting results and controversies have been reported. These conflicts highlight the requirement of further studies to provide valuable information within this area. In this systematic review, we summarize the current evidence on the co-infection with HTLV-1 among HIV-positive patients, its associated complications, and the impact on progression of AIDS.