Empirical evidence suggests that physical activity is related to less depressive moods. However, little is known about this association in the everyday life of older adults, limiting the ecological validity of prior findings. This study examined within-person associations between physical activity and depressive mood in older adults across 7 days. Moreover, the study tested the extent to which need-fulfillment can explain this association. The sample consisted of 68 adults aged 65 to 93 years. Physical activity was assessed objectively with accelerometers, whereas need-fulfillment and depressive mood were assessed at the end of each day using self-reports. Results from multilevel analysis suggest that daily physical activity was negatively related to daily depressive mood within persons. Although need-fulfillment did not explain the association between physical activity and depressive mood, it was a statistically significant predictor of daily depressive mood and even attenuated the effect of physical activity on depressive mood to nonsignificance.