BACKGROUND: Behavioral parent training (BPT) is recognized as a significant non-pharmacological treatment for attention-deficit/hyperactivity disorder (ADHD). However, there is limited empirical evidence of its effectiveness within the Chinese context, particularly for online BPT. This study evaluates the efficacy of an 8-session internet-based BPT (I-BPT) program in improving ADHD symptoms and mental health outcomes for children and parents, compared to routine clinical care (RCC). METHODS: Participants included 120 children with ADHD and their parents, recruited from the Wuhan Mental Health Centre between March and September 2023. They were randomly assigned to either the I-BPT+RCC (N = 60) or RCC (N = 60) group, with assessments conducted at baseline and after two months. Child-related outcomes measured were ADHD symptoms, depressive symptoms, anxiety symptoms and functional impairment. Parent-related outcomes included depressive symptoms, anxiety symptoms and parental stress. Analyses of covariance (ANCOVA) were used to compare the efficacy of I-BPT+RCC with the RCC alone. RESULTS: The I-BPT+RCC group showed significant improvements in total ADHD symptoms (F=6.84, P = 0.010), inattention (F=4.82, P = 0.030), hyperactivity/impulsivity (F=4.83, P = 0.030), and child depressive symptoms (F=8.56, P = 0.004) compared to the RCC group. Parental stress also significantly decreased in the I-BPT+RCC group (F=5.95, P = 0.016). CONCLUSIONS: Overall, the I-BPT significantly enhanced health outcomes for children with ADHD and their parents. These findings highlight the potential of I-BPT as an effective and accessible treatment option for a broader patient population.