Physical activity questionnaires are an important means to assess habitual physical activity. It remains unclear what questionnaires are used and whether they are appropriate for people with dementia who have impaired information recall but are also often largely sedentary. This scoping review aimed to identify and quantify the use of physical activity questionnaires within a dementia population. Eighteen studies met the inclusion criteria for this review. The majority of studies used questionnaires that were validated for use within an older adult population (e.g., Modified Baecke Questionnaire for the Elderly), though none had specifically been validated for use in people with dementia. Interestingly, just over half of the studies (N = 10, 55.6%) adapted the questionnaires from the original validated version by allowing a proxy to provide input to the responses. Future research needs to robustly validate the use of proxy-report measures of physical activity in people with dementia.