The physiological mechanisms underlying avian seasonal timing of reproduction, a life-history trait with major fitness consequences, are not well understood. Comparing individuals that have been selected to differ in their timing of breeding may prove to be a promising in studying these mechanisms, making selection lines a valuable tool. We created selection lines for early and late timing of breeding in great tits (Parus major) using genomic selection, that is selection based on multi-marker genotypes rather than on the phenotype. We took in nestlings (F(1 )generation) from wild broods of which the mother was either an extremely early ("early line") or extremely late ("late line") breeder. These chicks were then genotyped and, based on their "genomic breeding values" (GEBVs), we selected individuals for early and late line breeding pairs to produce the F(2 )generation in captivity. The F(2 )offspring was hand-reared, genotyped and selected to produce an F(3 )generation, which were then again genotyped and selected. This way we obtained laying dates in aviaries for F-1, F-2 and F(3 )birds. We studied the genetic response to the artificial selection and found increased genetic differentiation between the early and late reproducing selection lines over generations (F-1-F-3), indicated by both diverging GEBVs and increased fixation indices (F-ST). We studied the phenotypic response to selection for birds breeding in outdoor breeding aviaries. We found that early line birds laid earlier than late line birds, and this difference increased over the generations (F-1-F-3), with non-significant line effects for the F-1 and F-2, but highly significant line differences for the F-3. We also assessed whether there was correlated selection on two traits that are potentially part of the mechanisms underlying seasonal timing: the endogenous free-running period of the day/night clock (tau) and basal metabolic rate, but found no correlated selection. We have successfully created selection lines on seasonal timing in a wild bird species and obtained an instrument for future studies to investigate the physiological mechanisms underlying timing of breeding, and the genetic variation in these mechanisms, an essential component for evolutionary change in timing of reproduction. A free Plain Language Summary can be found within the Supporting Information of this article.