Males and females share many traits that have a common genetic basis, however selection on these traits often differs between the sexes leading to sexual conflict. Under such sexual antagonism, theory predicts the evolution of genetic architectures that resolve this sexual conflict. Yet, despite intense theoretical and empirical interest, the specific loci underlying sexually antagonistic phenotypes have rarely been identified, limiting our understanding of how sexual conflict impacts genome evolution and the maintenance of genetic diversity. Here, we identify a large effect locus controlling age at maturity in Atlantic salmon, an important fitness trait in which selection favours earlier maturation in males than females, and show it is a clear example of sex dependent dominance reducing intralocus sexual conflict and maintaining adaptive variation in wild populations. Using high density single nucleotide polymorphism (SNP) data across 57 wild populations and whole genome re-sequencing, we found that the vestigial-like family member 3 gene (VGLL3) exhibits sex-dependent dominance in salmon, promoting earlier and later maturation in males and females, respectively. There were also signs of spatially varying selection consistent with selection towards local optima. VGLL3, an adiposity regulator associated with size and age at maturity in humans, explained 39% of phenotypic variation, an unexpectedly large proportion for what is usually considered a highly polygenic trait. Such large effects are predicted under balancing selection from either sexually antagonistic or spatially varying selection. Our results provide the first empirical example of dominance reversal permitting greater optimisation of phenotypes within each sex, contributing to the resolution of sexual conflict in a widespread evolutionary trade-off between age and size at maturity in Altantic salmon. They also provide key empirical evidence for how variation in reproductive strategies can be maintained over large geographical scales.