Background Psychiatry rotation has been mandatory in the Japanese postgraduate residency system since 2020. Some psychiatry-related competency items are stipulated as mandatory for residents. The current study aimed to clarify whether psychiatry rotation affected residents’ subjective achievement of these competency items. Methods This longitudinal study was conducted among postgraduate residents who completed a rotation in the psychiatry department at Nagasaki University Hospital across two academic years (2020–2021). The survey was administered at the start and at the end of the psychiatry rotation. Residents evaluated their subjective understanding and confidence regarding initiating treatment for these competency items using a six-point Likert scale. The average scores for each item were compared between pre-rotation and post-rotation. Results In total, 99 residents (91.7%) responded to this survey. Residents had significantly higher scores at post-rotation compared with pre-rotation in all psychiatry-related competency items in both subjective understanding and confidence in initiating treatment. Additionally, strong effect sizes were found for many items. Conclusion Residents improved learning about psychiatry-related competency items through psychiatry rotation. This finding suggests that it is reasonable for psychiatry rotation to be mandatory in the current Japanese postgraduate residency system. The importance of psychiatry is likely to increase in both undergraduate and postgraduate medical education in the future. It is necessary to continuously update educational strategies to meet changing social needs over time. As this study was conducted at a single institution, a multi-center study is needed to expand the current findings.