Abstract Sexual size dimorphism (SSD) is predicted to vary across mating systems. A previous study examined a model of SSD in fishes as it relates to three mating system variables: probability of sperm competition, male territorial guarding, and male-male contest. I tested the ability of these variables to predict SSD in North American freshwater minnows, after controlling for phylogenetic effects by an independent contrasts method. Across 58 species only male territorial guarding was significantly related to SSD in a stepwise multiple regression. When tested for 26 genera and subgenera, both male territorial guarding and male-male contest were significant in the model. The concentrated-changes test revealed that character changes in SSD (from males the same size or smaller than females, to males larger than females) were more concentrated on branches with presence of male guarding (similar results were found for changes in SSD and presence of sperm competition), at the species and genus levels. Both comparative approaches demonstrated that male guarding and male-male contest variables are linked to SSD in minnows.