Advances in next-generation sequencing technologies and the development of genome-reduced representation protocols have opened the way to genome-wide population studies in non-model species. However, species with large genomes remain challenging, hampering the development of genomic resources for a number of taxa including marine arthropods. Here, we developed a genome-reduced representation method for the ecologically important marine copepod Calanus finmarchicus (haploid genome size of 6.34 Gbp). We optimized a capture enrichment-based protocol based on 2656 single-copy genes, yielding a total of 154 087 high-quality SNPs in C. finmarchicus including 62 372 in common among the three locations tested. The set of capture probes was also successfully applied to the congeneric C. glacialis. Preliminary analyses of these markers revealed similar levels of genetic diversity between the two Calanus species, while populations of C. glacialis showed stronger genetic structure compared to C. finmarchicus. Using this powerful set of markers, we did not detect any evidence of hybridization between C. finmarchicus and C. glacialis. Finally, we propose a shortened version of our protocol, offering a promising solution for population genomics studies in non-model species with large genomes.