ObjectiveThis study aimed to explore the effects of a mHealth-based relaxation program on anxiety and stress coping levels in mothers of children with cancer. MethodsThis randomized controlled, single-blind, parallel-group trial was conducted in the pediatric hematology-oncology unit of a university hospital. The study was completed by 50 mothers (intervention group: n = 25; control group: n = 25). The mothers in the intervention group participated in a mHealth-based relaxation program for eight weeks (including progressive muscle relaxation, guided imagery, and the four-leaf clover of mindfulness). Videos of the relaxation program were sent to mothers’ WhatsApp accounts. The mothers in the control group received routine care. The State-Trait Anxiety and Stress Coping Scales were administered during the first and eighth weeks. The registration number of this study on ClinicalTrials.gov is NCT05807295. ResultsMothers who participated in the relaxation program had lower anxiety levels and higher stress-coping scores than those in the control group (p < 0.05). ConclusionProgressive muscle relaxation exercises, guided imagery, and cognitive-based therapeutic approaches were effective in reducing mothers’ anxiety and stress coping. Practice implicationsPediatric oncology nurses can apply mHealth-based relaxation programs to support caregivers, particularly mothers. In this way, ensuring the continuity and integrity of care is easy.