What gives rise to sadism? While sadistic behavior (i.e., harming others for pleasure) is well-documented, past empirical research is nearly silent regarding the psychological factors behind it. We help close this gap by suggesting that boredom plays a crucial role in the emergence of sadistic tendencies. Across 9 diverse studies, we provide correlational and experimental evidence for a link between boredom and sadism. We demonstrate that sadistic tendencies are more pronounced among people who report chronic proneness to boredom in everyday life (Studies 1A-1F, N = 1,780). We then document that this relationship generalizes across a variety of important societal contexts, including online trolling; sadism in the military; sadistic behavior among parents; and sadistic fantasies (Studies 2-5, N = 1,740). Finally, we manipulate boredom experimentally and show that inducing boredom increases sadistic behavior (i.e., killing worms; destroying other participants' pay; Studies 6-9, N = 4,097). However, alternatives matter: When several behavioral alternatives are available, boredom only motivates sadistic behavior among individuals with high dispositional sadism (Study 7). Conversely, when there is no alternative, boredom increases sadistic behavior across the board, even among individuals low in dispositional sadism (Studies 8 and 9). We further show that excitement and novelty seeking mediate the effects of boredom, and that boredom not only promotes sadistic (proactive) aggression, but reactive aggression as well (Study 9). Overall, the present work contributes to a better understanding of sadism and highlights the destructive potential of boredom. We discuss implications for basic research on sadism and boredom, as well as applied implications for society at large. (PsycInfo Database Record (c) 2021 APA, all rights reserved).