Divorce and remarriage influence family relations, yet few studies explore changes in grandparenting due to family recomposition. We study variations in grandparental investment when the parents have children from several unions. Using nationally representative data of younger adults from the Generational Transmissions in Finland survey conducted in 2012 (sample n = 760 parents), we compare the grandchild care that parents report having received from their parents and parents-in-law. Results show that multipartner fertility is not associated with the amount of grandparental investment a parent receives from his or her own parents, but is associated with the investment received from mother's parents-in-law. Mother's parents-in-law are less likely to invest in grandchild sets which include step-grandchildren, compared to grandchildren living with their original parents. Fully biological grandchildren are 31% more likely to receive grandparental care compared to grandchild sets including step grandchildren. Thus the reduction in grandparental investment associated with step-grandchildren may also affect children from the new union.