Evidence suggests that omega-3 fatty acids are important for a variety of mental health outcomes and have been shown to improve both mood and behaviors. However, there is little consensus on whether omega-3 fatty acids are beneficial for reducing aggressive behaviors. The current study assesses the relationship between omega-3 fatty acids and aggression. A total of 73 effect sizes were calculated among 40 studies involving 7173 participants from both intervention and observational research designs. Effect sizes were separately meta-analyzed for two-group comparison studies (SMD = 0.20), pre-post contrast studies (ESsg = 0.62), and associational studies (r = −0.06), in the fixed-effect model. Results from the random-effects model also suggest a range of effects of omega-3 fatty acids on reducing aggression (SMD = 0.24; ESsg = 0.82; r = −0.09). Patterns in the relationship between omega–3s and aggression were additionally observed. Moderator analyses indicated that the effect of omega–3s on aggression is conditioned by how aggressive behaviors are measured, such as through self-report or parent/teacher surveys.