The operational sex ratio—the ratio of men to women in a given population—affects a range of social processes. The current research demonstrates that biased sex ratios (greater numbers of one sex than the other) influence fundamental aspects of people’s mating strategy. When the sex ratio was favorable (one’s own sex was in the minority), both sexes adopted strong sex-typical sociosexual orientations (relatively restricted for women; relatively unrestricted for men). When the sex ratio was unfavorable (one’s own sex was in the majority), both sexes shifted toward the orientation typically favored by the other sex: Women became more unrestricted and men became more restricted (Experiment 1). When the sex ratio was unfavorable (relative to favorable), participants also displayed greater aggression toward a romantically desirable (but not undesirable) same-sex partner (Experiment 2). Exploratory analyses suggested that the sex ratio effect was present for unprovoked aggression but not provoked aggression (given the exploratory nature of that analysis, the aggression effect should be considered with some caution). Findings suggest that people’s mating strategies are adaptively calibrated to contingencies within the local mating ecology.