<p>The streamflow data availability plays an important role in water resource management and hydraulic structure design such as flood and drought management, hydropower development, supporting irrigation systems, and ensuring water security but many watersheds in Ethiopia lack this essential data. This study was conducted to estimate continuous streamflow at the Muger watershed outlet using the inverse distance weighting (IDW) parameter regionalization technique with the SWAT model. The study aimed to predict the streamflow of ungauged watersheds. The specific objectives of the study were: (1) to assess the applicability of the SWAT model through calibration and validation in the gauged catchment, (2) identify the value of sensitive parameters at the gauged sub-watershed in the study watershed, (3) verify the IDW regionalization technique using cross-validation, (4) transfer the fitted value of sensitive parameters to the ungauged portion of the study watershed using IDW and rerun the SWAT model. Model performance values (R², NSE, PBAS) showed that the SWAT model and the IDW method were above the accepted range for all gauged sub-watersheds in the Muger watershed. Generally, the estimated annual streamflow of the Muger watershed is 260.65 m³/s. This result is important to designing the hydraulic structure and water resource projects; however, the model's uncertainties due to the input data and the model structure were accounted for in the analysis.</p>