The Wami-Ruvu River Basin is important for socio-economic activities in country such as water supply for Dar es Salaam and Morogoro cities, and major agricultural activities such as sugarcane irrigation at Mtibwa and Bagamoyo. Due to projected climate change and its impacts at global scale, it is important to understand future climate change impacts on water resources of Wami-Ruvu River basin. Rainfall and temperature are key variables for analysis of water resources and were used in this study.The statistical downscaling model (SDSM) was used to downscale the coarse global circulation model (GCM) to local scales by involving predictor predictand relationship. The predictor variables were selected based on partial correlation value (partial r) and significance value (p-value). For assessment of climate change, the baseline period was 30 years during 1961–1990. The baseline period was partitioned into two periods for SDSM calibration and validation: 1961–1975 and 1976–1990 respectively. In this case, ground stations and the U.S. National Center for Environmental Prediction (NCEP) reanalysis atmospheric data were used. During validation, the inbuilt scenario generator was used to generate simulated time series for five stations: Morogoro Maji, Ulaya, Ukaguru, Morogoro Airport, Dodoma Airport and Ruvu at Morogoro Rd. Brd. For precipitation, the SDSM's R2 (−) for the two periods ranged 0.07–0.20 and 0.21–0.63 respectively. The respective coefficient of correlation, r (−) ranged 0.03–0.05 and 0.46–0.80, indicating low to high performance of the SDSM. The respective R2 (−) values for temperature ranged: 0.42–0.5 and 0.6–0.98 respectively.The calibrated SDSM model was then used to downscale Global Circulation Model (GCM) scenario data to the local scales. The GCM used was HadCM3 where A2 and B2 scenarios were used. The climate change scenarios were determined using change factors. Results showed that for Wami-Ruvu basin the mean rainfall will change by −44–107%, −69–328% and 68–648% during 2020s, 2050s and 2080s for A2 scenario while by −37–117%, −56–199 and −76–346% respectively for B2 scenario as compared to the baseline period. In all cases, Ulaya and Morogoro Maji stations presented the lowest and highest extremes in the ranges. The downscaled and projected average monthly maximum temperature indicated increasing trend from 0.2 to 7.5 °C in 2020s–2080s time period. The minimum temperature showed decreasing trend from −0.4 to −1.5 °C during the same periods.These results indicate potential for floods or droughts occurrence in the basin, accordingly adaptation measures will be necessary.