Physical activity has been associated with a wide range of health benefits including long-term benefits for cognitive and brain health. Whether episodes of everyday physical activity are associated with immediate cognitive benefits remains unknown. The purpose of the current study was to examine whether episodes of physical activity, occurring over the course of participants' daily lives, are associated with short-term improvements in cognitive health. Participants completed a 9-day ecological momentary assessment protocol involving 5 daily assessments of self-reported physical activity and ambulatory cognitive assessments of processing speed and visuospatial working memory. Data were analyzed in a multilevel modeling framework to explore changes in performance on each task associated with physical activity during the period leading up to the assessment as well as individual differences in average frequency of physical activity. Results of MLMs indicated that engaging in physical activity during the period (~3.5hr) leading up to an assessment was associated with improvements in processing speed equivalent to 4 years of cognitive aging. Such improvements were observed for both light and moderate-to-vigorous physical activity levels. No association was observed for visuospatial working memory accuracy; however, response time during the working memory task reliably mirrored the association observed for processing speed. The short-term benefits were observed, particularly, for individuals with an overall higher frequency of reported physical activity. Our findings suggest that engaging in everyday physical activity of any intensity level may have short-term, acute benefits for cognitive health and point to new potential targets for intervention. NCT03240406.