Background. High levels of physical activity seem to positively influence health and cognition across the lifespan. Several studies have found that aerobic exercise enhances cognition and may prevent cognitive decline in the elderly. Nevertheless, the association of incidental physical activity (IPA) with health and cognition in elderly people has not been studied. Thus, the aim of this study was to evaluate the association between IPA levels and performance on a reading task during an event-related potentials (ERP) recording in elderly individuals. Methods. Spanish speaking participants (mean age = 66.7 years, SD = 4.3) with normal scores on psychometric and neuropsychological tests and normal values in blood analyses were included. Two groups were identified using a cluster analysis based on the scores of the Yale Physical Activity Scale (YPAS): Actives (n = 34), with high levels of IPA, and Passives (n = 34), with low levels of IPA. Actives showed significant higher Leisure activity (kcal/week; p = 0.002), and Vigorous (p < 0.0001) and Moving (p = 0.006) index scores of YPAS than Passives. ERPs were recorded while participants performed a reading task in which they had to identify grammatical errors. The sentences were manipulated with regard to (1) the gender agreement of the noun and adjective (i.e., the adjective’s gender either agreed or disagreed with the noun), and (2) the working memory (WM) load (i.e., the number of words between the noun and adjective in the sentence; high and low). Results. Actives showed earlier latencies of the left anterior negativity (LAN) than Passives in the low WM load condition. Moreover, their peak latencies were less variable than Passives’. Both groups showed an earlier latency of LAN in the low than high WM load condition, but this difference was greater in Actives. These latency patterns were also observed in the P600 ERP component. Conclusion/implication. Actives showed to be faster and less variable in their brain responses than Passives during morphosyntactic processing for sentence comprehension. Longer ERP latencies could be associated with speed of processing decline in aging. Our findings imply that a high level of IPA in older people could be a neuroprotective factor against cognitive decline associated with aging.