AbstractInbreeding depression refers to the reduced fitness of offspring produced by genetically‐related individuals and is expected to be rare in large, outbred populations. When it occurs, marked fitness loss is possible as large populations can carry a substantial load of recessive harmful mutations which are normally sheltered at the heterozygous state. Using experimental cross data and genome‐wide identity‐by‐descent (IBD) relationships from an outbred marine nine‐spined stickleback (Pungitius pungitius) population, we documented a significant decrease in offspring survival probability with increasing parental IBD sharing associated with an average inbreeding load (B) of 10.5. Interestingly, we found that this relationship was also underlined by a positive effect of paternal inbreeding coefficient on offspring survival, suggesting that certain combinations of parental inbreeding and genetic relatedness among mates may promote offspring survival. Our results demonstrate the potential for substantial inbreeding load in an outbred population and emphasize the need to consider fine‐scale genetic relatedness in future studies of inbreeding depression in the wild.