The sea louse (Caligus rogercresseyi) is an external parasite and considered one of the most important health problems in the salmon farming industry. Resistance to conventional chemical treatments has been demonstrated. Sufficient additive genetic variation has been determined to include selection for resistance to this parasite in Atlantic salmon breeding programs. The aim of this study was to perform a Genome Wide Association Study in order to dissect the genetic factors involved in the resistance to C. rogercresseyi, one of the most important species of sea lice in the Chilean salmon farming. 2628 Atlantic salmon smolts, which had been experimentally infested with C. rogercresseyi, were genotyped using a 50K SNP array. Genome Wide Association Analysis was conducted using a polygenic model. A heritability of 0.12 for resistance to this louse species was estimated using genomic information. This result was consistent with estimates from previous studies which used pedigree records in the same population. Only one SNP, located on chromosome 21, was significant at a local level, explaining 0.5% of the phenotypic variance and 4% of the genomic heritability for sea lice resistance. This SNP is located in an intronic region of a predicted gene which codes for Collagen alpha-1. Our results suggest that resistance to C. rogercresseyi can be considered a polygenic trait, controlled by many variants of relatively small effect. Thus the incorporation of genomic information through genomic selection could be the most appropriate approach for breeding purposes. Statement of relevance Caligus resistance has a polygenic genetic architecture.