ABSTRACT: Importance:Parents of children with serious illness, such as cancer, experience high stress and distress. Few parent-specific psychosocial interventions have been evaluated in randomized trials. Objective:To determine if individual- or group-based delivery of a novel intervention called Promoting Resilience in Stress Management for Parents (PRISM-P) improves parent-reported resilience compared with usual care. Design, Setting, and Participants:This parallel, phase 2 randomized clinical trial with enrollment from December 2016 through December 2018 and 3-month follow-up was conducted at Seattle Children's Hospital. English-speaking parents or guardians of children who were 2 to 24 years old, who had received a diagnosis of a new malignant neoplasm 1 to 10 weeks prior to enrollment, and who were receiving cancer-directed therapy at Seattle Children's Hospital were included. Parents were randomized 1:1:1 to the one-on-one or group PRISM-P intervention or to usual care. Data were analyzed in 2019 (primary analyses from January to March 2019; final analyses in July 2019). Interventions:The PRISM-P is a manualized, brief intervention targeting 4 skills: stress management, goal setting, cognitive reframing, and meaning making. For one-on-one delivery, skills were taught privately and in person for 30 to 60 minutes approximately every other week. For group delivery, the same skills were taught in a single session with at least 2 parents present. Main Outcomes and Measures:Participants completed patient-reported outcome surveys at enrollment and at 3 months. Linear regression modeling evaluated associations in the intention-to-treat population between each delivery format and the primary outcome (Connor-Davidson Resilience Scale scores, ranging from 0 to 40, with higher scores reflecting greater resilience) and secondary outcomes (benefit finding, social support, health-related quality of life, stress, and distress) at 3 months. Results:In total, 94 parents enrolled, were randomized to 1 of the 3 groups, and completed baseline surveys (32 parents in one-on-one sessions, 32 in group sessions, and 30 in usual care). Their median (interquartile range) ages were 35 to 38 (31-44) years across the 3 groups, and they were predominantly white, college-educated mothers. Their children had median (interquartile range) ages of 5 to 8 (3-14) years; slightly more than half of the children were boys, and the most common cancer type was leukemia or lymphoma. One-on-one PRISM-P delivery was significantly associated with improvement compared with usual care in parent-reported outcomes for resilience (?, 2.3; 95% CI, 0.1-4.6; P?=?.04) and for benefit finding (?, 0.5; 95% CI, 0.2-0.8; P?=?.001). No significant associations were detected between either platform and other parent-reported outcomes. Conclusions and Relevance:When delivered individually, PRISM-P was associated with improved parent-reported resilience and benefit finding. This scalable psychosocial intervention may help parents cope and find meaning after their child receives a diagnosis of a serious illness. Trial Registration:ClinicalTrials.gov identifier: NCT02998086.