Understanding the impact of demographic and ecological factors on the strength of sexual selection is pivotal to predict eco-evolutionary dynamics in populations facing environmental change. Density is expected to show pervasive intra- and interspecific variation in natural populations, but its effect on sexual selection remains largely unexplored. We tested experimentally the effect of density on standardized metrics of sexual selection in male and female red flour beetles. Specifically, we explored density effects during mating by manipulating the size of mating groups and the mating arena. We found that higher densities imposed by a larger group size amplified the opportunity for pre-copulatory sexual selection in females and, to a smaller extent, also in males. Moreover, in males, we detected a higher opportunity for post-copulatory sexual selection in larger groups. By contrast, higher densities imposed by differences in the size of the mating arena resulted in a steeper male sexual selection gradient. Overall, these data suggest that density is an important environmental factor contributing to the tremendous variation in the strength of sexual selection within and among species. Read the free Plain Language Summary for this article on the Journal blog.