The structural integrity of hydropower dams is paramount for sustainable energy production and downstream safety. Traditional inspection methods, such as core drilling, are destructive, localized, and often fail to capture the spatial heterogeneity within the concrete. This study evaluates the efficacy of the multi-channel analysis of surface waves (MASW) method—a non-destructive seismic technique—for assessing the stiffness and identifying potential defects within the Se San 4 Hydropower Dam in Gia Lai, Vietnam. We conducted MASW surveys along two strategic profiles: the dam crest surface and the internal inspection tunnel. By inverting Rayleigh wave dispersion curves, we derived two-dimensional shear wave velocity (V_s) profiles and subsequently calculated shear modulus (G) distributions to map the stiffness of conventional vibrated concrete (CVC) and roller-compacted concrete (RCC) layers. The results reveal a distinct stratification in the dam body. The surface survey identified a top layer of CVC/asphalt with V_s ranging from 1,500 to 2,500 m/s, overlying a stiffer RCC layer exhibiting V_s values between 2,500 and 4,000 m/s. Conversely, the tunnel survey recorded significantly higher velocities in the RCC core (4,000–5,500 m/s), reflecting better confinement and material quality at depth. Crucially, the study detected specific low-velocity anomalies (V_s < 2,000 m/s) and reduced stiffness zones at depths of 17–30 m along the survey lines. These anomalies may indicate areas of potential water seepage, variable concrete density, or natural construction lift joints inherent to the RCC layering process. This research demonstrates that MASW is a rapid and cost-effective tool for the periodic structural integrity monitoring of large-scale hydraulic structures, providing critical baseline data for maintenance and risk mitigation in tropical operating environments.