AIMS AND OBJECTIVES: Quarantine during the COVID-19 pandemic resulted in longer-term sedentary behaviours and mental health problems. Our study aimed to evaluate the impact of the Otago exercise programme (OEP) on physical function and mental health among elderly with cognitive frailty during COVID-19. BACKGROUND: Lockdowns and restrictions during the COVID-19 pandemic result in longer-term sedentary behaviours related disease and mental problem. Older people with cognitive frailty are more vulnerable to be influenced. Timely intervention may achieve better outcomes, OEP exercise was designed as a balance and muscle-strengthening programme for elderly people. DESIGN: A parallel-group, assessor-blinded randomised controlled trial was performed according to CONSORT guidelines. METHODS: This study was conducted from July 2020 to October 2020 among 62 elderly people with cognitive frailty from a nursing home. Participants were randomly divided into an OEP group (n = 31) or a control group (n = 31). Both groups received sleep- and diet-related health education. The OEP group also received a 12-week group exercise programme. The Five Times Sit to Stand Test (FTSST), Berg Balance Scale (BBS), and Timed Up and Go Test (TUGT) were used to assess physical function. The Geriatric Depression Scale-15 (GDS-15) and the 12-Item Short Form Health Survey Mental Component Summary (SF-12 MCS) were used to assess mental health. Outcomes were measured at 6 and 12 weeks. RESULTS: Physical function and mental health were similar in the two groups at baseline. At 12 weeks, the OEP group (difference in change from baseline: FTSST, -2.78; TUGT, -3.73; BBS, 2.17; GDS-15, -0.72; SF-12 MCS, 2.58; all p < .001) exhibited significantly greater improvements than the control group (difference in change from baseline: FTSST, 1.55; TUGT, 1.66; BBS, -0.10; GDS-15, 1.07; SF-12 MCS, -5.95; all p < .001). CONCLUSION: Our findings showed the OEP group had better physical function and mental health outcomes than the control group. OEP can be used to improve the physical and mental function among elderly people with cognitive frailty during the COVID-19 pandemic. RELEVANCE TO CLINICAL PRACTICE: Otago exercise program intervention programmes should be implemented to improve physical function for cognitive frailty elderly to reduce the harm of longer-term sedentary behaviours, and to ruduce depression symptom and improve mental health, particularly during COVID-19 pandemic period.