Understanding the genomic basis of adaptation is critical for understanding evolutionary processes and predicting how species will respond to environmental change. Spinner dolphins in the eastern tropical Pacific (ETP) present a unique system for studying adaptation. Within this large geographic region are four spinner dolphin ecotypes with weak neutral genetic divergence and no obvious barriers to gene flow, but strong spatial variation in morphology, behavior, and habitat. These ecotypes have large population sizes, which could reduce the effects of drift and facilitate selection. To identify genomic regions putatively under divergent selective pressures between ecotypes, we used genome scans with 8,994 RADseq SNPs to identify population differentiation outliers and genotype-environment association outliers. Gene ontology enrichment analyses indicated that outlier SNPs from both types of analyses were associated with multiple genes involved in social behavior and hippocampus development, including fifteen genes associated with the human social disorder autism. Evidence for divergent selection on social behavior is supported by previous evidence that these spinner dolphin ecotypes differ in mating systems and associated social behaviors. In particular, three of the ETP ecotypes likely have a polygynous mating system characterized by strong pre-mating competition among males, whereas the fourth ecotype likely has a polygynandrous mating system characterized by strong post-mating competition such as sperm competition. Our results provide evidence that selection for social behavior may be an evolutionary force driving diversification of spinner dolphins in the ETP, potentially as a result of divergent sexual selection associated with different mating systems.