This study examined the effects of physical activity on working memory in older adults using both behavioral and neuroelectric measures. Older adults were assigned to either a higher or lower physical activity group, and event-related potentials were recorded during assessments of a modified Sternberg task. The results indicated that older adults in the higher physical activity group exhibited shorter response times, independent of the working memory load. Enhanced P3 and N1 amplitudes and a decreased P3 latency were observed in the higher physical activity group. These findings suggested that physical activity facilitates working memory by allocating more attentional resources and increasing the efficiency of evaluating the stimulus during the retrieval phase as well as engaging more attentional resources for the early discriminative processes during the encoding phase of a working memory task.