Fecundity selection, acting on traits enhancing reproductive output, is an important determinant of organismal body size. Due to a unique mode of reproduction, mating success and fecundity are positively correlated with body size in both sexes of male-pregnant Syngnathus pipeﬁsh. As male pipeﬁsh brood eggs on their tail and egg production in females occurs in their ovaries (located in the trunk region), fecundity selection is expected to affect both sexes in this species, and is predicted to act differently on body proportions of males and females during their development. Based on this hypothesis, we investigated sexual size dimorphism in body size allometry and vertebral numbers across populations of the widespread European pipeﬁsh Syngnathus typhle. Despite the absence of sex-speciﬁc differences in overall and region-speciﬁc vertebral counts, male and female pipeﬁsh differ signiﬁcantly in the relative lengths of their trunk and tail regions, consistent with region-speciﬁc selection pressures in the two sexes. Male pipeﬁsh show signiﬁcant growth allometry, with disproportionate growth in the brooding tail region relative to the trunk, resulting in increasingly skewed region-speciﬁc sexual size dimorphism with increasing body size, a pattern consistent across ﬁve study populations. Sex-speciﬁc differences in patterns of growth in S. typhle support the hypothesis that fecundity selection can contribute to the evolution of sexual size dimorphism.