The efficacy of our newly developed self-help parenting program on children’s externalizing behavior was recently shown. The present study investigated its efficacy on parental outcomes, which is understudied in clinical trials. Using a randomized controlled trial, 110 parents were allocated to an intervention condition (either with or without telephonic support) or a waitlist condition. Outcomes included the Parenting Practices Interview (PPI; positive and negative parenting practices), Parenting Sense of Competence Scale (PSOC), Parenting Stress Index (PSI) and Eyberg Child and Behavior Inventory - Problem Scale (ECBI-P; the number of child behaviors that parents perceive as troublesome). All measures were collected at baseline (T0) and post intervention (after 15 weeks, T2); PPI and ECBI-P were also collected at week 8 (T1) and 28 (T3). Using longitudinal multi-level regression analyses, we compared the intervention and waitlist condition at T2. Furthermore, the support and no support conditions were exploratively compared at T2 and T3. Comparing intervention to waitlist, results showed medium-sized effects on PPI and ECBI-P scores and a small-sized effect on PSI scores, and no differences between the conditions on PSOC scores. No differences were found between the support and the no-support condition. The results indicate that our self-help program improved parenting practices and parents’ perception of child behavior, in addition to previous findings on child outcomes. This confirms that self-help parenting programs may be a promising alternative to face-to-face programs, although future studies should focus on determining for whom and in which settings which parenting programs are most helpful..