Behavioural parent training is an evidence-based intervention for children with attention-deficit/hyperactivity disorder (ADHD), but little is known about the extent to which initial benefits are maintained. This meta-analytic review investigated longer-term (i.e., more than two months post-intervention) child and parental outcomes of behavioural parent training for children with ADHD. We searched for randomized controlled trials and examined ADHD symptoms, behavioural problems, positive parenting, negative parenting, parenting sense of competence, parent-child relationship quality, and parental mental health as outcomes. We included 27 studies (31 interventions; 238 effect sizes), used multilevel random-effects meta-analyses for between- and within-group comparisons (pre-intervention to follow-up and post-intervention to follow-up), and explored twelve predictors of change. Between pre-intervention and follow-up (M = 5.3 months), we found significant small-to-moderate between-group effects of the intervention for ADHD symptoms, behavioural problems, positive parenting, parenting sense of competence and parent-child relationship quality, and small-to-large within-group improvements for all outcome domains except parent-child relationship quality. There were few significant changes between post-intervention to follow-up. The smaller effects in the between- compared to within-group analyses reflect no deteriorations in the intervention conditions between post-intervention and follow-up, indicating the added value of investigating within-group changes. Most within-group effect sizes indicated sustained improvements from post-intervention to follow-up (92% sustained, 4% fade-out, 4% sleeper). There were seven significant predictors of change, including stronger reductions in ADHD symptoms of girls and behaviour problems of younger children. In contrast with studies on short-term effects, we found no differences between masked and unmasked outcomes on ADHD symptoms at follow-up. We conclude that behavioural parent training has longer-term benefits for children’s ADHD symptoms and behavioural problems, and for positive parenting behaviours, parenting sense of competence and quality of the parent-child relationship.