ObjectiveOrthognathic surgery is a complex orofacial surgery that can significantly impact occlusal function and effect nutritional and quality of life parameters. This study aimed to evaluate alterations in dietary intake, chewing function, physical activity, and oral health–related quality of life of patients undergoing orthognathic surgery. Research Methods and ProceduresIn this prospective longitudinal study, the assessments were conducted at: preoperatively (T0) and postoperative first week (T1), second week (T2), first month (T3), and third month (T4) between May 2021 and March 2023. Sociodemographic questionnaire, 24-h dietary recall record, chewing ability form, International Physical Activity Questionnaire, and Oral Health Impact Profile-14 (OHIP-14) was applied at face-to-face interviews. ResultsSeventy eligible orthognathic surgery patients were evaluated, and 37 patients (52.8%) completed this study. Energy and fat intake significantly decreased from T0 to T1 (P < 0.001) and returned to basis by T4 (P = 0.015). Fiber intake was found to be lowest at T1 and T2 compared with other time points (P < 0.001). Chewing ability showed a deterioration and then improvement; however, patients still had difficulties chewing hard foods at T4. The OHIP-14 increased at T2 and T3 from T0 (P < 0.001 and P = 0.021, respectively) and showed a significant improvement at T4 (P < 0.05). ConclusionThe findings indicate a temporary decline in nutritional intake and chewing ability with subsequent recovery by the third month postsurgery. These changes, along with the trends in oral health-related quality of life, underscore the need for tailored nutritional and functional rehabilitation programs following orthognathic surgery.