Sexual size dimorphism (SSD), a difference in body size between the sexes, occurs in many animal species. Although the larger sex is often considered invariable within species, patterns of selection may result in interpopulation variation or even reversal of SSD. We evaluated correlations between latitude and female body size, male body size, and relative body size (male body size/female body size) in 22 populations (ranging from 37°N to 49°N) of sea-run masu salmon (Oncorhynchus masou) that spawn in rivers along the Sea of Japan coast. Male size and the relative body size increased with latitude, but female size did not correlate with latitude. In addition, increase in male size with latitude was sufficient to result in a reversal of SSD, the switch-point being around 45°N. We suggest that the positive correlation between latitude and male size is due to increasing operational sex ratios or sexual selection on sea-run male body size that result from sex-biased patterns of anadromy. In conclusion, our study provides the first example of predictable geographic variation in SSD shaped by apparent patterns of sexual selection.