Many organisms are expanding their ranges in response to changing environmental conditions. Understanding the patterns of genetic diversity and adaptation along an expansion front is crucial to assessing a species’ long-term success. While next-generation sequencing techniques can reveal these changes in fine detail, ascribing them to a particular species can be difficult for organisms that live in close association with symbionts. Using a modified restriction-site associated DNA sequencing (RAD-Seq) protocol to target coral DNA, we collected 595 coral-specific single nucleotide polymorphisms (SNPs) from 189 colonies of the invasive coral Oculina patagonica from across the Spanish Mediterranean coast, including established core populations and two expansion fronts. Surprisingly, populations from the recent northern expansion are genetically distinct from the westward expansion and core populations, and also harbor greater genetic diversity. Generalized linear models relating genetic structure to environmental variables suggest that temperature appears to have driven adaptation along the northern expansion, but not toward the west. Tests for selection found two candidate genes associated with temperature in the northward expansion, but none in the west. We found no evidence of local adaptation to artificial substrate, which has been proposed for explaining the rapid spread of O. patagonica, suggesting that this coral is simply an opportunistic colonizer of free space made available by coastal habitat modifications. Together, these results suggest that unique genetic variation and adaptation to local temperatures along the northern expansion front may have facilitated the poleward range expansion of O. patagonica in the western Mediterranean.