Despite the ubiquitous nature of parasitism, the general effects of how parasitism alters the outcome of host species interactions such as competition, mutualism, predation, and reproduction remain unknown. Using a meta-analysis of 178 studies, we examined how the outcomes of diverse species interactions differed between parasitized and non-parasitized hosts. We also evaluated how the effects of parasitism on species interactions varied geographically with latitude. Overall, parasitism had relatively large deleterious effects on the outcome of host species interactions. However, there was considerable variation among interactions in these outcomes, with reproduction severely negatively affected, marginal effects on competition, and muted effects on predation – the latter results emanating because of the surprising tendency of parasitism to frequently reduce the effects of competition and predation. The effects of parasites did not differ between macro- and microparasites, nor did the shared evolutionary histories of hosts and parasites have an effect. Parasites had detrimental effects on the outcomes of species interactions near the equator, but more variable effects at temperate and higher latitudes. These results highlight the need to better understand how parasitism can affect the multitude of complex species interactions that structure biological communities.