In children with autism spectrum disorder (ASD), problem behaviors play a dysfunctional role, causing as much difficulty with daily living and adjustment as the core symptoms. If such behaviors are not effectively addressed, they can result in physical, economic, and psychological issues not only for the individual but also for family members. We aimed to develop and evaluate the feasibility of a mobile app-assisted parent training program for reducing problem behaviors in children with ASD. This open-label, single-center, randomized controlled trial was conducted among parents of children with ASD aged 36-84 months. Participants were recruited from the Department of Psychiatry at Seoul National University Hospital. Participants were randomly assigned (1:1) by a blinded researcher. Randomization was performed using a stratified block randomization (with a block size of 4). Parents in the intervention group completed the mobile app-assisted parent training program at home over a 12-week period. They continued to receive their usual nondrug treatment in addition to the mobile app-assisted parent training program. The control group continued to receive their usual nonpharmaceutical treatment for 12 weeks without receiving the parent training program intervention. The primary outcome measure was the median change in the Korean Child Behavior Checklist (K-CBCL) scores from before to after the intervention. Lower scores on the K-CBCL indicated a decrease in overall problem behavior. Between November 9, 2022, and December 8, 2022, 64 participants were enrolled. Overall, 42 children (intervention group median age: 49, IQR 41-52.5 months; control group median age: 49, IQR 42-58 months) of the participants joined the program. The intervention group included 20 (48%) participants and the control group included 22 (52%) participants. In the intervention group, the K-CBCL total scores showed a decrease after the intervention, with a median difference of -0.5 (95% CI -4.5 to 3). Pervasive developmental disorder scores also showed a decrease, with a median difference of -2.1 (95% CI -8.5 to 2.5). However, there was no significant difference in Clinical Global Impression-Severity of Illness scores after the intervention for both the control and intervention groups. Scores on the Korean version of the Social Communication Questionnaire showed a further decrease after the intervention in the intervention group (median difference -2, 95% CI -4 to 1). Caregivers' stress evaluated using the Korean Parenting Stress Index Fourth Edition-Short Form did not show any significant differences between the control and intervention groups. There were no adverse events related to study participation. The findings demonstrated the feasibility of using mobile devices for evidence-based parent training to reduce problem behaviors in children with ASD. Mobile devices' accessibility and flexibility may provide a viable alternative for offering early intervention for problem behaviors in children with ASD. CRIS KCT0007841; https://cris.nih.go.kr/cris/search/detailSearch.do?&seq=23112.