Earthfill dams are widely used because they can be constructed under diverse geological and geographical conditions and can utilize locally available materials. However, seepage and slope instability remain critical safety concerns in dam design. The configuration and thickness of the zones within the dam body, particularly the core, can significantly affect seepage control and slope stability. This study evaluates three alternative zoned earthfill dam configurations for Jlantah Dam, Indonesia: the existing vertical-core design (Alternative 1), an inclined-core design (Alternative 2), and a diaphragm-type design (Alternative 3). Seepage and slope stability analyses were performed using GeoStudio SEEP/W and SLOPE/W under maximum flood level (MFL), minimum water level (MWL), and rapid drawdown (RD) conditions, considering non-seismic, Operating Basis Earthquake (OBE), and Maximum Design Earthquake (MDE) loading. The allowable seepage discharge was set at 0.0617 m³/s, corresponding to 1% of the average river discharge. The calculated seepage discharges for Alternatives 1–3 ranged from 0.00906 to 0.0197 m³/s, with Alternative 3 producing the lowest values of 0.0179, 0.00906, and 0.0169 m³/s under MFL, MWL, and RD conditions, respectively. Under non-seismic loading, the calculated Factor of Safety (FS) values ranged from 1.912 to 2.877, exceeding the minimum requirement of 1.3 for all alternatives. Under OBE loading, the FS values ranged from 1.277 to 1.683, satisfying the minimum requirement of 1.2. However, under MDE loading, the FS values ranged from 0.523 to 0.742, which are below the minimum requirement of 1.0 for all alternatives. Overall, Alternative 3 provides the most effective seepage control, whereas Alternative 1 provides the highest slope stability. The results demonstrate that dam-body configuration significantly influences the balance between seepage control and slope stability and should therefore be carefully considered in the design and evaluation of zoned earthfill dams.