OBJECTIVES: To determine the psychological impact of the COVID-19 pandemic on people with and without an inflammatory rheumatic disease and establish whether psychological flexibility buffers this impact. METHODS: From online surveys in the general Dutch population in 2018 and during the peak of the COVID-19 pandemic in 2020, we analysed data of people with (index group, n = 239) and without (control group, n = 1821) an inflammatory rheumatic disease. Worry, stress, mental well-being (SF-36) and psychological flexibility levels were subjected to covariate-adjusted analyses of variance or linear regression analyses. RESULTS: During the peak of the COVID-19 pandemic in 2020, as compared with the control group, the index group was more worried about getting infected with the virus (partial η2=0.098; medium effect) and more stressed (partial η2=0.040; small effect). However, as compared with data acquired in 2018, the level of mental well-being during the COVID-19 pandemic peak was not lower in both groups. Levels of psychological flexibility did not moderate associations of group or year with mental well-being. CONCLUSIONS: Although patients with an inflammatory rheumatic disease were more worried and stressed during the peak of the COVID-19 pandemic, their level of mental well-being was not reduced, which may have prevented us from finding a buffering effect of psychological flexibility. Overall, our results suggest that the psychological impact of the COVID-19 pandemic in patients with inflammatory rheumatic disease is modest, which could imply that common education and health care will do for most patients.