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Aim: To model and forecast the morbidity incidence of Human African Trypanosomiasis (HAT) in Nigeria using the Autoregressive Integrated Moving Average (ARIMA) time series approach. Study Design: A retrospective longitudinal time series study design was adopted. Place and Duration of Study: The study was conducted in Nigeria using annual reported cases of Human African Trypanosomiasis from 1960 to 2024. Methodology: Human African Trypanosomiasis (HAT), also known as sleeping sickness, is a vector-borne parasitic disease caused by Trypanosoma brucei species and transmitted by infected tsetse flies. Annual time series data of HAT infection cases in Nigeria covering the period 1960–2024 were analyzed. The time series properties of the data were examined using graphical plots and the Phillips–Perron unit root test to determine stationarity. The results indicated that the series was integrated of order two, I(2). Following the Box–Jenkins modeling procedure, several ARIMA (p, d, q) models were estimated, and model selection was based on diagnostic and goodness-of-fit criteria. The ARIMA (3,2,3) model emerged as the most appropriate model for capturing the dynamics of HAT incidence in Nigeria. Results: The selected ARIMA (3,2,3) model was dynamically stable and accounted for 76.31% of the variability in HAT infection cases. Forecasts generated for an 11-year period (2025–2035) revealed a consistent decline in the number of reported HAT cases, with an average projected annual incidence of approximately 146 cases. The downward trend suggests that sustained interventions—including improved disease surveillance, enhanced treatment accessibility, and strengthened vector control strategies—may have contributed to the reduction in infection rates. Conclusion: The findings of this study indicate a declining trajectory of Human African Trypanosomiasis in Nigeria over the forecast period. Nevertheless, continuous surveillance, application of predictive time series modeling, intensified vector control measures, and sustained public health awareness campaigns remain essential to consolidate gains and prevent potential resurgence of the disease.