IntroductionWorldwide, the prevalence of degenerative diseases such as dementia and mild cognitive impairment (MCI) is increasing with population ageing and increasing life expectancy. Both conditions share modifiable risk factors. Physical inactivity is one of these modifiable risk factors, and research points to the protective effect of physical activity on the incidence of dementia and MCI. However, this association tends to change according to type, intensity, frequency, duration and volume of physical activity. Furthermore, it remains unclear which of these characteristics offers the greatest protective effect. Therefore, this study aims to evaluate the impacts of different types, intensities, frequencies, duration and volume of physical activity on dementia and cognitive decline in older adults.Methods and analysisThe search will be carried out from October 2023, using the following databases: PubMed, Embase, Scopus, CINAHL and Web of Science. Cohort studies with a follow-up time of 1 year or longer that have investigated the incidence of dementia and/or MCI in older adults exposed to physical activity will be included. There will be no limitations on the date of publication of the studies. Studies published in English, Spanish or Portuguese will be analysed. Two researchers will independently screen the articles and extract the data. Any discrepancies will be resolved by a third reviewer. Association measures will be quantified, including OR, HR, relative risk and incidence ratio, with a 95% CI. If the data allow, a meta-analysis will be performed. To assess the methodological quality of the selected studies, the Grading of Recommendations, Assessment, Development and Evaluations instrument, and the Downs and Black instrument to assess the risk of bias, will be used.Ethics and disseminationEthical approval is not required. The results will be submitted for publication in a peer-reviewed journal.PROSPERO registration numberCRD42023400411.