Research has shown that social support influences parenting, and yet few studies have reported how different dimensions of social support influence outcomes in a group-based parenting intervention for parents with young children. We examined the influence of social support source, type, satisfaction, and access barriers on parental outcomes in the “Growing Up Happily in the Family” program targeting parents with children aged 0–5. The sample was 256 mainly at-risk (87.2%) and non-at-risk parents participating in 49 program groups in local social services in three Autonomous Communities of Spain. Social support variables were examined as predictors of pre- post change scores in parents’ child-rearing attitudes, sense of parental competence, and parenting stress using hierarchical linear regression analyses. Results showed significant contributions of social dimensions to lack of empathy, belief in corporal punishment, parental satisfaction, dysfunctional interaction, and perception of the child as difficult. The direction of the effects varied depending on the social support dimension considered. While formal support use was associated with positive changes in parenting stress, informal support predicted positive changes in child-rearing attitudes. Support involving tangible and positive social interaction and the cumulation of perceived barriers to service access were related to negative changes in child-rearing attitudes and satisfaction with the role. In conclusion, efforts should be made to promote the dimensions of support that are most useful in increasing the effectiveness of evidence-based parenting programs.