Abstract: Based on inclusive fitness theory, amounts of grandparental investment can be predicted to reflect the probability to share common genes with grandchildren. Adoption may represent a special case, however, yet grandparental investment in adoptive children has often been theoretically misconstrued and little investigated. Here, we study for the first time whether grandparental child care provision is unequally distributed between biological, adopted and step-offspring. Using the Survey of Health, Ageing and Retirement in Europe (SHARE) (n = 26,183 grandmother-adult child and 19,339 grandfather-adult child dyads) and Generations and Gender Surveys (GGS) (n = 15,168 adult child-grandmother and 12,193 adult child-grandfather dyads) and, we find that grandparents were less likely to provide care to step-grandchildren than to other types of grandchildren, but no significant difference in grandparental childcare channelled towards adopted and biological grandchildren. These findings were present in both datasets and for both grandmothers and grandfathers, after several potentially confounding factors were taken into account. The step-child disadvantage is in line with kin selection theory, while the congruent amounts of care provided towards of adopted and biological grandchildren is likely due to similar psychological attachment between the two groups, as well as to overcompensation and greater need in the case of adopted children. The study provides new evidence of how genetic relatedness and psychological attachment may shape kin investment in contemporary societies.