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Research suggests that physical activity may play a role in preserving cognitive function in older adulthood. However, the exact nature, direction, and magnitude of observed associations remain unclear. The current study utilized a microlongitudinal design to repeatedly assess cognitive function and physical activity across five days. Two studies examined relationships between physical activity, physical fitness, and cognitive function among community-dwelling older adults. The first study examined associations between baseline performance in a measure of everyday cognition and multiple measures of physical activity and physical fitness. Bivariate analyses revealed that objectively measured physical activity of moderate-to-vigorous intensity, repeated chair stand time and 6-minute walk distance were significantly associated with everyday cognition. After adjusting for covariates in a multiple regression model, physical activity was not significantly associated with everyday cognition. However, a composite physical fitness score created from 6-minute walk distance and repeated chair stand time was significantly associated with DECA, and the full model accounted for 38% of the variance in baseline DECA performance. The second study investigated within- and between-person relationships between daily physical activity and cognitive function. Study participants wore an activity monitor and completed a battery of cognitive assessments for five days. Multilevel modeling analyses indicated that same-day total number of steps was significantly associated with better visual speed of processing but not everyday cognition, or inductive reasoning.. Time spent in moderate-to-vigorous activity was not significantly associated with same-day cognitive performance in any domain. However, previous-day moderate-to-vigorous physical activity was significantly associated with better inductive reasoning and speed of processing the following day, after controlling for age, gender and physical fitness. Time spent in moderate-to-vigorous activity explained 16% of the within-person variability in speed of processing. Physical fitness and age did not explain significant variability in between-person cognitive function. Results obtained in the present study varied according to how physical activity and cognition were operationalized and measured. Associations between physical activity and cognition were more evident with moderate-to-vigorous activity, as opposed to total activity, and an acute temporal relationship was suggested, with better cognitive performance following engagement in moderate-to-vigorous physical activity. Results also indicated that within-person fluctuations in domains of cognitive performance were positively associated with physical activity, and were more pronounced with cognitively complex tasks that were timed.