ABSTRACT: High latitude environments represent climate-related challenges, imposing a limit on species distributions. By elucidating how marine species adjust their reproductive strategy to cope with climatic variation, we can hope to better predict the resilience of populations to environmental change. We addressed this question with a large-scale field study of a marine fish with a central role in the North-East Atlantic coastal ecosystem: the two-spotted goby Pomatoschistus flavescens. We developed a multi-faceted approach to understand how the reproductive strategy and life-history of this fish changes with a latitudinal climatic gradient. Six populations were surveyed along a gradient of ten degrees of latitude reaching close to the northern end of the species range, combining environmental, physiological, phenotypic and demographic data, with a genomics investigation of population connectivity. Northern fish experienced colder environments of growth and reproduction than their southern counterparts, occurred at lower adult density, and had lower body condition. At high latitude a new life-history strategy emerged, where adults reproduced as two rather than one-year olds. Genomics analyses unveiled a clear genetic structure, indicating a break in connectivity between Skagerrak and the Norwegian coast further north, consistent with findings in other marine species. However, our genomic analyses did not provide clear evidence of adaptation to ecological variables, leaving open questions about the roles of adaptation and plasticity in latitudinal shifts in reproductive strategy. The study highlights the complex life-history adaptations to high latitudes in a marine species with high gene flow, and the importance of sea temperature on fish reproductive strategy.