The study determined rainfall onset and cessation dates, and length of rainy season (LRS) in the Oti River Basin (ORB) using historical data (1981–2010) and future (2021–2050) model data. Historical data were from meteorological stations, Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) and National Aeronautics and Space Administration Prediction of Worldwide Energy Resource (NASA POWER) while ensemble of eight models were acquired from Coordinated Regional Climate Downscaling Experiment (CORDEX-Africa). An evaluation of the models was performed based on correlation, standard deviation and root-mean-square error (RMSE). The onset, cessation and LRS were analyzed for trends using the Mann–Kendall test and Sen’s slope estimator. An evaluation of the performance of the models on rainfall revealed strong correlation (r > 0.9), with standard deviation < 2.0 and RMSE < 0.75 whereas for temperature a strong correlation (r ≥ 0.99), standard deviation < 1.25 and RMSE < 0.25 were also recorded. Results also showed that onset date could increase from 8 May (128 days) to about 24 May (144 days) in the future under the Representative Concentration Pathways (RCPs) emission scenarios. Cessation date which was on 29 October (302 days) in the historical period could decrease to 7 October (280 days) under both RCP4.5 and RCP8.5 scenarios in future. The LRS would decrease from 173 days to about 136 days under both emission scenarios. These anticipated variations in onset and cessation dates, and LRS are amongst the important rainfall characteristics that could affect future agricultural viability and planning, and the availability of water in the ORB.