ImportancePsychoeducational interventions (PEIs), which provide both information and emotional and psychosocial support, may address the unmet needs of the caregivers of adolescent and young adult patients with cancer.ObjectiveTo explore the effects of an instant messaging–delivered PEI on anxiety, depression, quality of life (QOL), and coping and determine whether the intervention reduces caregivers’ unmet needs.Design, Setting, and ParticipantsThis randomized clinical trial using an intention-to-treat protocol was conducted from April 1 to September 14, 2022, in a tertiary cancer hospital in China and included caregivers of patients diagnosed with cancer at age 15 to 39 years recruited using convenience sampling.InterventionCaregivers were allocated 1:1 using a randomized block scheme to the intervention or control group. The intervention group received a 5-week PEI and usual care, whereas the control group received only usual care. The PEI was delivered through articles and calls through an instant messaging application.Main Outcomes and MeasuresThe primary outcomes were changes in the levels of caregivers’ anxiety and depression, measured using the 7-Item Generalized Anxiety Disorder Scale and the Patient Health Questionnaire 9, respectively. The secondary outcomes were changes in the levels of caregivers’ QOL, coping, and unmet needs.ResultsOf the 160 participants, 92 (57.5%) were male; mean (SD) age was 40.27 (8.33) years. Compared with the control group, the intervention group had significantly greater reduction in severity of anxiety (B = −3.231; 95% CI, −4.746 to −1.716; P < .001) and depression (B = −3.253; 95% CI, −5.052 to −1.454; P < .001), smaller reduction in QOL (B = 13.574; 95% CI, 0.488-26.661; P = .04), and greater reduction in unmet needs (B = −12.136; 95% CI, −18.307 to −5.965; P < .001) after the intervention. Twelve weeks after baseline, the intervention group demonstrated a significantly greater reduction only in severity of anxiety (B = −1.890; 95% CI, −3.382 to −0.397; P = .01).Conclusions and RelevanceIn this randomized clinical trial of a mobile instant messaging–delivered PEI, caregivers’ unmet needs, anxiety, and depression decreased significantly and QOL declined at a significantly slower rate immediately after the intervention in the intervention group compared with the control group. A sustained effect on anxiety was observed 12 weeks after baseline.Trial RegistrationChinese Clinical Trial Registry identifier: ChiCTR2200055951