BACKGROUND: The World Health Organization (WHO) recommends a minimum of 150 minutes of moderate physical activity per week. Adherence to these recommendations is difficult to assess. OBJECTIVE: We aimed to evaluate the validity of self-reported physical activity in mild vascular cognitive impairment (mVCI) and whether physical activity was associated with cognitive status, by using baseline data from a randomized controlled trial. METHODS: A hundred and four subjects with mVCI were included (mean age 72 years; 51% women). Subjects underwent neurological, physical, and comprehensive neuropsychological assessments. Adherence to WHO physical activity recommendations was evaluated using both self-reported information and objective measures (accelerometry). RESULTS: There was poor agreement (kappa = 0.106) between self-report of following WHO recommendations and actually fulfilling them according to accelerometry. Only 16.6% of participants reported following WHO recommendations and displayed compatible values according to the accelerometer. Participants whose accelerometry values confirmed adherence to WHO recommendations had better performance in a global measure of cognition, attention, and mental speed processing. In multiple regression analyses, education and accelerometry values in accordance with WHO recommendations were independently associated with the global measure of cognition, attention, and processing speed, controlling for sex, age, and depressive symptoms. Accelerometry results were not associated with memory and executive functions. CONCLUSION: In this sample of mVCI subjects, self-reported physical activity displayed poor agreement with accelerometry values, suggesting that objective measures of physical activity are preferable. Physical activity (performed, at least, according to WHO recommendations) was associated with better cognitive performance overall.