Purpose: This study aimed to identify the effects of a 12-week interprofessional simulation program, operated between February 2020 and January 2021, on the patient safety competencies of healthcare professionals in Switzerland. Methods: The simulation training was based on 2 scenarios of hospitalized patients with septic shock and respiratory failure, and trainees were expected to demonstrate patient safety competencies. A single-group before and after study was conducted after the intervention—simulation program, using a measurement tool (the Health Professional Education in Patient Safety Survey) to measure the perceived competencies of physicians, nurses, and nursing assistants. Out of 57 participants, 37 answered the questionnaire surveys 4 times: 48 hours before the training, followed by post-surveys at 24 hours, 6 weeks, and 12 weeks after the training. The linear mixed effect model was applied for the analysis. Results: Four components out of 6 perceived patient safety competencies improved at 6 weeks but returned to a similar level before training at 12 weeks. Competencies of “communicating effectively,” “managing safety risks,” “understanding human and environmental factors that influence patient safety,” and “recognize and respond to remove immediate risks of harm” are statistically significant both overall and in the comparison between before the training and 6 weeks after the training. Conclusion: Interprofessional simulation programs contributed to developing some areas of patient safety competencies of healthcare professionals, but only for a limited time. Interprofessional simulation programs should be repeated and combined with other forms of support, including case discussions and debriefings, to ensure lasting effects.