Parents should differentially allocate resources to the production of offspring of either sex depending on their expected fitness return. In sexually promiscuous females, offspring sex ratio should be affected by the sexual attractiveness of biological fathers because sons, but not daughters, will benefit from inheriting genes for sexual attractiveness. Females may acquire benefits for the offspring if the extra-pair male is of superior genetic quality as compared to the social mate, if it carries compatible genes or genes that enhance offspring genetic diversity. If sexually selected ornaments reflect male quality, extra-pair offspring should be more frequently males and of higher quality compared to their half-siblings. Furthermore, the probability of extra-pair offspring to be male should increase with an increasing difference in sexual ornamentation between the extra-pair and the social mate. In this study, we tested if barn swallow offspring sex ratio depends on paternity and on sexual ornamentation of the extra-pair father, and whether paternity predicts offspring phenotypic quality. The results partially fulfilled our prediction. We did not provide support for the hypothesis of a differential sex allocation to the production of male offspring by promiscuous females, both at the individual and at the within-brood level. Moreover, the difference in the ornamentation of the extra-pair and the social male did not affect the sex ratio of individual offspring. However, in accordance with the good genes and the genetic compatibility hypotheses, extra-pair offspring were of superior quality as compared to their half-sibling, in terms of body size and feather growth.