OBJECTIVE: This study aimed to evaluate the effect of magnesium supplementation on anxiety, depression and sleep quality of patients after openheart surgery. MATERIALS AND METHODS: Based on a clinical trial, 60 candidates for open-heart surgery were separated into control and intervention groups. In the intervention group, 500 mg of magnesium was administered daily in the form of two 250-mg magnesium oxide tablets for five days, and routine care was performed in the control group. Patients’ demographic information and assessment based on the Hospital Anxiety and Depression Scale (HADS) and Petersburg Sleep Quality Inventory (PSQI) were documented at the beginning of the study, after surgery and endotracheal intubation, and at the end of the study. Data were statistically analysed using Stata software version 13. RESULTS: After the intervention, the mean level of magnesium in the intervention group was significantly higher (p=0.001). At the end of the study, the mean level of anxiety and depression was significantly lower in the intervention group than the control group (p=0.007), moreover, mean sleep quality improved in the intervention group (8.3 ± 2.1) compared to the control group (10.3 ± 2.0) (p=0.001). CONCLUSION: Due to the effects of magnesium supplementation in patients after heart surgery, it is recommended that this supplement be included in the treatment plan in order to prevent anxiety and depression in these patients after surgery and improve their quality of sleep.