Background: Obesity, laparoscopic surgeries, and pain are factors that negatively affect postoperative diaphragm functions. We aimed to evaluate the diaphragm function using ultrasonography (USG) and spirometry in patients who underwent bariatric surgery and to investigate the effect of postoperative pain score on diaphragm function.Methods: Diaphragm inspiratory amplitude (DIA), expiratory velocity, and inspiratory velocity values were measured using USG in the preoperative period, in the recovery room on postoperative day 1 in 45 patients who underwent laparoscopic bariatric surgery under general anesthesia. Preoperative and postoperative spirometry measurements were performed.Results: In all patients, spirometry and USG measurements showed a significant decrease in recovery on postoperative day 1 compared with preoperative values. A negative correlation was found between the 1st-day pain score measured at rest and movement and DIA values measured in the sniffing position (p = 0.049 and 0.024, respectively). A significant correlation in the same direction was found between the duration of surgery and anesthesia and the change in the DIA being measured at normal respiration on postoperative day 1 compared with the preoperative values (p = 0.004 and p = 0.005, respectively).Conclusions: In our study, diaphragm and spirometry measurements significantly decreased in the postoperative period compared with the baseline.The Clinical Trial Registration number: NCT04828408.