A substantial proportion of parents whose child is diagnosed with a life-threatening illness experience high levels of distress that can lead to long-term mental health difficulties. This can affect the child's recovery. To evaluate the efficacy of an acceptance and commitment therapy-based group intervention, delivered using videoconferencing, in reducing posttraumatic stress symptoms (PTSS) in these parents. This study was a randomized clinical trial of an intervention for parents with elevated acute stress symptoms. It was a single-site study conducted in a tertiary pediatric hospital in Australia. Parents of children aged 0 to 18 years admitted for a life-threatening illness or injury to the oncology, cardiology, or pediatric intensive care departments were eligible. Participants were screened for eligibility within the first month after diagnosis or admission and then were randomized to the intervention group or the waiting list control group 4 to 10 months after diagnosis or admission. Recruitment commenced January 2014, and final postintervention follow-up was completed in February 2018. Data analysis was performed from July to September 2018. Treatment was a psychological acceptance and commitment therapy-based group therapy program called Take a Breath, which consisted of a 6-session parent-mediated psychological intervention delivered via online videoconferences over the course of 8 weeks. Waiting list control participants received treatment as usual and were offered the intervention 3 months after randomization. The primary outcome was PTSS, as measured by the Posttraumatic Stress Disorder Checklist-Version 5 (total score range, 0-80, with higher scores indicating greater symptom severity). The PTSS was measured both before and immediately after the intervention. Changes in psychological skills taught within the intervention were also evaluated, including acceptance, mindfulness, values-based living, and psychological flexibility. Of 1232 parents who were assessed for eligibility, 313 were randomized; 161 were allocated to the waiting list control group, and 152 were allocated to the intervention group. Of those allocated, 44 parents in the waiting list group and 37 in the intervention group completed the postintervention questionnaire and were analyzed (81 participants total; mean [SD] age, 37.17 [6.43] years). Sixty-five participants (80.2%) were women, 48 participants (59.3%) were married, and 40 participants (49.4%) lived in rural or regional areas, or in a different state. In addition, 24 parents (29.6%) were in the cardiology illness group, 32 parents (39.5%) were in the oncology group, and 25 parents (30.9%) were in the pediatric intensive care unit group. The intervention group demonstrated significantly greater improvements in PTSS compared with the waiting list group (Cohen d = 1.10; 95% CI, 0.61-1.59; P = .03). The mean Posttraumatic Stress Disorder Checklist-Version 5 scores decreased from 31.7 (95% CI, 27.0-36.4) to 26.2 (95% CI, 21.8-30.7) in the waiting list control group and from 23.3 (95% CI, 18.6-28.1) to 17.8 (95% CI, 13.8-21.8) in the intervention group. The findings of this study support the use of acceptance and commitment therapy to reduce PTSS in parents of very ill children, regardless of diagnosis. These findings also suggest that a brief, group format using a videoconferencing platform can be used effectively to access hard-to-reach populations, particularly fathers and caregivers living in nonmetropolitan areas. Australian New Zealand Clinical Trials Registry Identifier: ACTRN12611000090910.