With a length of 950 km, Karun River is the longest river in Iran. In this study, we aimed at application of Sentinel-3B satellite altimetry data as well as Sentinel-1 and Sentinel-2 satellite imagery for the estimation of Karun River discharge and validation with the in-situ data. Knowing that Level-2 altimetry data are not reliable for rivers and shallow waters, we opted to re-track the waveforms of Level-1B Sentinel-3B mission data and to test several re-tracking techniques for this purpose. The results showed that the threshold algorithm, with threshold of 90%, improves the accuracy of the time series of water level by 7. 05% and increases the correlation with the in-situ gauge data by 12. 7% as compared with those obtained via Level-2 data based on OCOG that was identified as the optimum re-tracker in this case. Next, from the estimated time series of the river’ s water level, the time series of Karun River discharge were evaluated in order to constitute our discharge estimation based on Sentinel-3B satellite altimetry data, which further to be compared with the discharge that we calculated using satellite imagery of Sentinel-1 and Sentinel-2, while taking the in-situ data as the benchmark. The river’ s discharge time series obtained from the altimetry data resulted in RMSE value of 852. 31 m 3 /s, NSE coefficient of 0. 19 m 3 /s, and correlation of 62. 40% with the in-situ river discharge time series. On the other hand, the river discharge time series obtained from satellite imagery of Sentinel-1 mission resulted in RMSE value of 165. 06 m 3 /s, NSE coefficient of 0. 94 m 3 /s, and correlation of 97. 12%, and Sentinel-2 mission the RMSE value 264. 23 m 3 /s, NSE coefficient of 0. 81 m 3 /s, and the correlation of 97. 32% with in-situ data. The overall results of this study indicates that various Copernicus satellites missions have good potentiality for Karun River discharge monitoring.