When females mate with more than one male, sexual selection acts both before and after mating. The interaction between pre- and post-mating episodes of selection is expected to be context dependent, but few studies have investigated how total sexual selection changes under different ecological conditions. We examined how population density mediates the interaction between pre- and post-mating sexual selection by establishing replicate populations of the horned dung beetle Onthophagus taurus at low, medium, and high densities, and using microsatellite-based parentage analyses to measure male fitness. We found that mating success and fertilization success were positively correlated at all three densities, but the strength of the correlation decreased with increasing density. We also found a shift from negative to positive linear selection on testes mass as density increased, and opposing selection on weapons and testes at high densities. These patterns suggest that the importance of post-mating processes increases with increasing population density, which reduces the selective advantage of weapons for pre-mating contest competition, and increases the selective advantage of large ejaculates for post-mating sperm competition. We expect that density-dependent selection on testes mass has contributed to the phenotypic variation observed between natural populations of O. taurus that differ in density.