Introduction: Human T-cell leukemia virus type 1 (HTLV-1) has worldwide distribution and is considered endemic in southwestern Japan. HTLV-1 infection has been associated with adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) besides other diseases. Objective: This cross-sectional study aimed to investigate the prevalence, risk factors, and molecular characterization of HTLV-1, among the world´s largest population of Japanese immigrants and their descendants outside Japan, in São Paulo, Southeast Brazil, as well as to analyze the phylogenetic relationship among isolates of HTLV-1. Methods: From July to December 2017, 2,139 individuals from five Japanese associations were interviewed and submitted to blood collection. All serum samples were first tested for the presence of anti-HTLV-1/2 antibodies by ELISA and then peripheral blood from individuals with positive serological results were analyzed for the presence of HTLV-1 5ʹLTR proviral DNA. Partial sequencing of the 5ʹLTR region of HTLV-1 proviral DNA was performed by Sanger. Results: The prevalence of HTLV-1 infection was 5.1% (95% CI 4.2-6.0). In the multiple logistic regression model, HTLV-1 infection was associated with age 45 years, female sex, first- and second-generation Japanese immigrants, and having sexual partners with a history of blood transfusion. The phylogenetic analysis revealed that all HTLV-1 were classified as Cosmopolitan (1a) subtype. Of them, 47.8% were classified as Transcontinental (A) subgroup and 52.2% as belonging to the Japanese (B) subgroup. Although most HTLV-1-infected patients were asymptomatic (97.3%), the blurred vision was associated with HTLV-1 infection. Conclusion: The high prevalence of HTLV-1 infection found in this study population and especially the intrafamily and interfamily HTLV-1 transmission presents an urgent need for preventive and control responses of this infection in Brazil.