Understanding how environmental gradients shape genetic variation is fundamental to evolutionary ecology. Isophya rizeensis, an endemic bush cricket distributed along the Fırtına Valley of Türkiye, exhibits striking altitudinal color polymorphism, making it an ideal system for studying local adaptation. Using genome-wide RAD-seq data from 71 individuals across and altitudinal gradient (450–2300 m elevation), we identified $92,048$ polymorphic loci, including 1,113 putatively adaptive SNPs. Despite low genome-wide differentiation (Fst < 0.05), adaptive loci exhibited greater divergence, suggesting selection-driven genetic structuring. Genome-wide association analyses identified 101 SNPs significantly correlated with altitude, with bidirectional allele frequency shifts indicating disruptive selection. Discriminant analysis further revealed three major-effect loci differentiating color morphs, which were also significantly associated with elevation. Our findings illustrate the interplay between selection, gene flow, and local adaptation, highlighting how environmental heterogeneity structures genomic variation in montane ecosystems. These results provide insights into the genetic basis of phenotypic divergence and contribute to understanding biodiversity persistence under changing environmental conditions.