Mental imagery and relaxation are noninvasive methods and can easily be used by the patient. Accordingly, this study aimed to detect the effect of guided mental imagery (GI) and progressive muscle relaxation (PMR) on sedation in patients undergoing noninvasive mechanical ventilation (NIV). This clinical trial study was carried out in the Shahid Modarres Hospital in 2022 and encompassed 80 patients with COVID-19 undergoing NIV, who were assigned to two intervention and control groups. The sedation survey scale was used to collect the required data. Before the intervention, the two groups completed the questionnaires, and then the relaxation program was presented during four sessions for the intervention group. This study included 80 patients with a mean age (± SD) of 59.59 ± 9.27 years. The Quade nonparametric ANCOVA revealed the significant impact of intervention on the post values of Total score (p-value < 0.001), Tolerance (p-value < 0.001), Calmness (p-value < 0.001), Ventilator synchrony (p-value < 0.001), face relaxation (p-value < 0.001), Consciousness (p-value = 0.009) and Awakeness (p-value = 0.020). The study findings demonstrated the effectiveness of relaxation program in promoting sedation in patients undergoing NIV. Accordingly, intensive care unit (ICU) nurses at our institution are encouraged to use PMR and GI to sedation of patients under NIV.