Background The aim of this Flemish study (Dutch speaking part of Belgium) is to evaluate the efficacy of an intervention based on the social learning theory for foster parents who take care of a foster child (ages 3–12) with externalizing problem behaviour (a borderline or clinical score on the externalizing broad‐band or on one of the externalizing problems scales of the Child Behaviour Checklist), with respect to breakdown, foster children's externalizing problems and foster mothers' parenting stress. Methods A randomized controlled trial was used to compare, at baseline, post‐intervention and three‐month follow‐up, an intervention group (n = 30) with a treatment as usual control group (n = 33) consisting of foster parents of new placements recruited via a routine screening or on‐going placements signed up by their foster care worker. The intervention consists of 10 home sessions. Results The intervention had significant small short‐term en large long‐term effects on externalizing problems and medium short‐term and long‐term effects on parenting stress. No significant effects were found on the breakdown rate. Conclusions Giving the evidence for its effects on foster children's externalizing and foster mothers' parenting stress, implementation of this foster parent intervention might increase the effectiveness of foster care.