Background: Cancer regularly disrupts health and developmental trajectories in adolescents and young adults (AYAs). Parents have been shown to have a substantial impact on the health and cancer survivorship activities of AYA patients in the form of symptom management. However, no randomized controlled trial has evaluated a coping support intervention (CSI) program for parents of AYAs with cancer aged 18 to 40 years. Patients and Methods: From November 30, 2012, to August 29, 2016, parents of AYAs with hematologic malignancies were randomized in a phase III controlled trial (1:1 ratio, stratified sampling) to either the research-based CSI AYA-Parents group (CSI group; n=82) or the standard care (SC) group (n=70). CSI consisted of 5 sessions to achieve the enhancement of parental adaptive coping as the primary outcome (per the adaptive coping scale of the 28-item Brief COPE, a validated multidimensional self-assessment-questionnaire recommended for clinical cancer research). Measures of adaptive coping, depression, and mental health were collected at pre-CSI (measurement date T1), at the end of the intervention sessions (measurement date T2), and at follow-up (3 months). We calculated mean change scores in outcomes and estimated intervention effect sizes (Cohen’s d) for changes from T1 to T2/T3, with 0.2 indicating a small effect, 0.5 a medium effect, and 0.8 a large effect. All statistical tests were 2-sided. Results: In the intention-to-treat analysis, the CSI group significantly improved their adaptive coping compared with the SC group (95% CI, 0.30–2.54; P=.013; d=0.405), whereas adaptive coping in the SC group deteriorated. The CSI group also experienced a significant decrease in depressive symptoms and improved mental health with clinical significance (95% CI, –1.98 to –0.30; P=.008; d=0.433, and 95% CI, –0.19 to 3.97; P=.074; d=0.292, respectively). Sensitivity analyses confirmed the robustness of the main intention-to-treat analysis. Conclusions: CSI improved effectively adaptive coping and depression in parents of AYAs with hematologic malignancies. It may represent a novel family-based approach in AYA oncology care.