This retrospective cohort study examined the incidence of post-COVID psychiatric disorders in older adults according to hospitalization status and SARS-CoV-2 variant period in Japan. Claims data, COVID-19 case-related information, and vaccination records were obtained from three Japanese municipalities. We identified individuals aged ≥65 years who had COVID-19 or other respiratory tract infection (RTI) between March 2021 and December 2022. Participants were categorized into non-hospitalized and hospitalized patients, and the study period was divided into the Alpha (March to May 2021), Delta (June to December 2021), Omicron BA.1/BA.2 (January to June 2022), and Omicron BA.5 (July to December 2022) periods. Modified Poisson regression analyses were performed to estimate the incidence rate ratios (IRRs) for the occurrence of psychiatric disorders (organic mental disorders, psychotic disorders, mood disorders, anxiety disorders, and insomnia) three months after COVID-19 (reference: other RTI). For overall psychiatric disorders, we analyzed 19,489 non-hospitalized patients (COVID-19: 6,728, Other RTI: 12,761) and 2925 hospitalized patients (COVID-19: 1,036, Other RTI: 1889). When compared with other RTI cases, COVID-19 cases had significantly lower IRRs for overall psychiatric disorders in both non-hospitalized (IRR: 0.59, P < 0.001) and hospitalized cases (IRR: 0. 83, P = 0.045) during the Omicron BA.1/BA.2 period, but only in non-hospitalized cases (IRR: 0.45, P < 0.001) during the Omicron BA.5 period. The incidences of the individual post-COVID psychiatric disorders varied according to disorder type, hospitalization status, and SARS-CoV-2 variant period. These findings provide a foundation for further research to explore these variations and improve the provision of psychiatric care in future epidemics.