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Rensch's rule is a biological rule on allometrics, concerning the relationship between the extent of sexual size dimorphism (SSD) and which sex is larger. Across species within a lineage, size dimorphism will increase with increasing body size when the male is the larger sex, and decrease with increasing average body size when the female is the larger sex. This correlation between SSD and body size is hypothesized to be a result of an increase in male-male competition in larger species, a result of limited environmental resources, fueling aggression between males over access to breeding territories and mating partners. The rule was proposed by the evolutionary biologist Bernhard Rensch in 1950.

Examples of phylogenetic lineages that appear to follow this rule include primates, pinnipeds, and artiodactyls.