Inbreeding and the associated increase in homozygosity and potential accumulation of deleterious alleles may reduce fitness in a process known as inbreeding depression. Mechanisms to mitigate reproduction between close relatives, ranging from pre-mating mate choice to post-mating gamete selection, have evolved across taxa. In external fertilizers like Atlantic salmon (Salmo salar), where females have limited control over paternity, mechanisms of inbreeding avoidance can be expected to evolve at the gamete level. Philopatric Atlantic salmon may run the risk of breeding between relatives, particularly in small populations, and frequent escapes from aquaculture settings are augmenting the chances of adults from the same sibling cohort overlapping in the wild, raising potential ecological and sustainability concerns. Moreover, the presence of inbreeding avoidance mechanisms between full siblings in externally fertilising fish is currently untested. This study tested post-mating inbreeding avoidance mechanisms in domesticated Atlantic salmon. In a paired breeding design, we compared sperm motility parameters in sibling and non-sibling ovarian fluid, and assessed fertilisation and hatching success, growth rate, and paternity in sperm competition trials between sibling and non-sibling males. Sperm activated in ovarian fluid of sibling females showed lower values of motility-related parameters and led to an average of 18% reduction in fertilisation rates in the resulting crosses. Furthermore, offspring from sibling crosses were smaller before the onset of sexual maturation. However, we found no difference in survival rates between sibling and non-sibling cross offspring. Besides, when sperm from sibling and non-sibling males were competing simultaneously for the same egg batch, we found no influence of this on paternity. Our findings reveal the presence of post-mating inbreeding avoidance at the gamete level in Atlantic salmon, but its effects are limited in competitive scenarios. Our results have implications for salmonid conservation and aquaculture, where small natural or closed domestic strains may both display a degree of inbreeding. Mating between escaped domestic and wild salmonids could favour admixed over wild or feral crosses, if an inbreeding avoidance mechanism is present, although this remains to be tested in more outbred crosses.