Adaptation to different salinities can drive and maintain divergence between populations of aquatic organisms. Anadromous and stream ecotypes of threespine stickleback (Gasterosteus aculeatus) are an excellent model to explore the genetic mechanisms underlying osmoregulation divergence. Using a parapatric pair of anadromous and stream stickleback ecotypes, we employed an integrated genomic approach to identify candidate genes important for adaptation to different salinity environments. Quantitative trait loci (QTL) mapping of plasma sodium concentrations under a seawater challenge experiment identified a significant QTL on chromosome 16. To identify candidate genes within this QTL, we first conducted RNA-seq and microarray analysis on gill tissue to find ecotypic differences in gene expression that were associated with plasma Na+ levels. This resulted in the identification of ten candidate genes. Quantitative PCR analysis on gill tissue of additional Japanese stickleback populations revealed that the majority of the candidate genes showed parallel divergence in expression levels. Second, we conducted whole-genome sequencing and found five genes that are predicted to have functionally important amino acid substitutions. Finally, we conducted genome scan analysis and found that eight of these candidate genes were located in genomic islands of high differentiation, suggesting that they may be under divergent selection. The candidate genes included those involved in ATP synthesis and hormonal signalling, whose expression or amino acid changes may underlie the variation in salinity tolerance. Further functional molecular analysis of these genes will reveal the causative genetic and genomic changes underlying divergent adaptation.