Despite the effectiveness of positive behavior support (PBS) in reducing challenging behaviors, the availability of PBS for individuals with intellectual disabilities is limited in many countries including the Netherlands. Training care staff supporting individuals with intellectual disabilities in PBS may be a way to improve the provision of PBS. We aimed to explore the preliminary effectiveness of a PBS training for staff in reducing challenging behaviors of individuals with intellectual disabilities. Using a one group, double pretest-posttest design, 24 staff members involved in the care of 11 adult individuals with intellectual disabilities and challenging behaviors participated. We assessed changes in challenging behaviors and quality of life of the individuals, in staff self-efficacy in dealing with challenging behaviors, and in the use of restraints, using staff rated questionnaires, structured interviews, and medical files. At posttest, we found significant reductions in challenging behaviors, improved quality of life, and increased staff self-efficacy, but no changes in the use of restraints. In contrast, no significant changes on any of the measures appeared between the two pretests. These findings suggest that a staff training in PBS may be effective for reducing challenging behaviors in individuals with intellectual disabilities.