Signals of natural selection can be quickly eroded in high gene-flow systems, curtailing efforts to understand how and when genetic adaptation occurs in the ocean. This long-standing, unresolved topic in ecology and evolution has renewed importance because changing environmental conditions are driving range expansions that may necessitate rapid evolutionary responses. One example occurs in Kellet’s whelk (Kelletia kelletii), a common subtidal gastropod with a ~ 40-60 day pelagic larval duration that expanded their biogeographic range northward in the 1970s by over 300 kilometers. To test for genetic adaptation, we performed a series of experimental crosses with Kellet’s whelk adults collected from their historical (HxH) and recently expanded range (ExE), and conducted RNA-Seq on offspring that we reared in a common garden environment. We identified 2,770 differentially expressed genes (DEGs) between 54 offspring samples with either only historical-range (HxH offspring) or expanded-range (ExE offspring) ancestry. Using SNPs called directly from the DEGs, we assigned samples of known origin back to their range of origin with unprecedented accuracy for a marine species (92.6 and 94.5% for HxH and ExE offspring, respectively). The SNP with the highest predictive importance occurred on triosephosphate isomerase (TPI), an essential metabolic enzyme involved in cold stress response. TPI was significantly upregulated and contained a non-synonymous mutation in the expanded range. Our findings pave the way for accurately identifying patterns of dispersal, gene flow, and population connectivity in the ocean by demonstrating that experimental transcriptomics can reveal mechanisms for how marine organisms respond to changing environmental conditions.