The erosion wear due to raindrop plays a vital role in the service life of wind turbine components, especially blades. Furthermore, the rain impact angle and the environmental conditions have a major impact on erosion wear. In the present work, the rain erosion performance of glass fiber reinforced polyester (GFRP) blade material in offshore and onshore environmental conditions has been studied. A whirling arm test rig was created and used to conduct the erosion tests on both offshore and onshore conditions. To simulate the offshore and onshore environmental conditions, experiments were carried out using both fresh and saltwater. The trail runs were carried out for time duration (30–90 min) at different impact angles (0—90 deg) by running the whirling arm tester at different impact velocities (30–70 m/s). During the experimentation, the impact velocity varied from 30 to 70 m/s. The impact angle was taken as 0 deg, 30 deg, 45 deg, 60 deg, and 90 deg, respectively. The results showed that erosion wear in the offshore conditions was more in all the cases as compared to onshore conditions. Furthermore, the maximum mass loss was at 45-deg impact angles for both conditions, which was further endorsed by Scanning Electron Microscope (SEM) analysis. Very little work has been reported on the optimization of erosion wear response of Offshore and Onshore wind turbines by using different design of experiment techniques. The erosion testing reveals that the GFRP blades exhibit a ductile erosion mechanism, which was further explained by using three-dimensional surface plots with the help of Image J software.