Challenging behaviours are common and disabling consequences of acquired brain injury (ABI), causing stress for close-others and disrupting community integration. Positive Behaviour Support (PBS) interventions have support from case studies as a means of reducing these behaviours, but controlled trials are lacking. This study aimed to evaluate, with a randomised waitlist-controlled trial, the efficacy of a 12-month PBS intervention, termed PBS+PLUS, in reducing challenging behaviours after ABI. Participants included 49 individuals with ABI (PBS+PLUS Intervention and Waitlist Treatment-as-usual groups) and their close-others. The design was a randomised waitlist-controlled trial with 12-month follow-up. PBS+PLUS involved setting collaborative goals toward a more meaningful life with the individual with ABI and addressing barriers to achieving these, including challenging behaviours, through skill-building, restructuring environments and executive strategies. The primary outcome measure was the Overt Behaviour Scale (OBS). The Challenging Behaviour Self-Efficacy Scale (CBSES) was a secondary outcome, assessing close-others' self-efficacy in addressing challenging behaviour. Measures were completed at baseline and every 4 months. Data analyses involved mixed-effects and negative binomial regressions examining change over time by group. The PBS+PLUS Intervention group included 24 participants and the Waitlist group 25 participants. On the OBS, the Intervention group showed a significant reduction in challenging behaviour over the 12-month intervention. However, the Waitlist group showed similar improvement on the OBS over the 12-month waitlist period. The Waitlist group was not denied an intervention from other providers during this period. Gains continued for 8 months post-intervention. The PBS+PLUS intervention resulted in significantly greater gains in close-others' confidence in addressing challenging behaviours on the CBSES, relative to those in the Waitlist group, who showed no such gains. Although we cannot say this intervention is more effective than any other, the study did show that PBS+PLUS can result in significant and sustained reductions in challenging behaviour in individuals with severe ABI and increased confidence of close-others in addressing these difficult behaviours. #ACTRN12616001704482.