Trauma-related social, emotional and behavioral difficulties (SEBD) are common among children in foster care and are the primary reason for placement breakdown. SEBD in foster children – and especially in the context of unstable and troubled relationships with both foster and biological parents - affects the child’s future functioning and has substantial cost implications in terms of public service utilization. The aim of this study was to assess the utility and perceived effectiveness of the 18-week Incredible Years parenting program (IYPP) which was delivered, on an exploratory basis, to both biological and foster parents (including kinship and non-relative care) of 23 foster children (aged 3–10 years). Biological and foster parent pairs (n = 46) were assessed at pre-intervention and at 6-month follow up, using measures of child SEBD, parenting stress, competencies, and quality of child-parent/carer relationships. One-to-one interviews and a focus group were also undertaken with a subset of biological parents (n = 12), foster carers (n = 11) and Social Work clinicians (n = 5) who delivered the program; the findings were analyzed using grounded theory. Both biological and foster parents reported statistically significant improvements in child SEBD, parent–child relationships, and in parenting stress and competencies. The qualitative findings highlighted further benefits for families, such as an increased number of access visits between biological parents and children and improved relationships with Social Work clinicians. Several factors were identified as important when implementing the IYPP with foster children, including: potential difficulties in engaging both foster and biological parents within the Social Work infrastructure; making appropriate adaptations to program principles, and integrating delivery with a trauma-informed approach. These findings contribute to the growing body of evidence that the IYPP could add value to the standard training and supports for foster parents, children and biological parents.