The positive effects of physical activity on mood are well documented in cross-sectional studies. To date there have been only a few studies analyzing within-subject covariance between physical activity and mood in everyday life. This study aims to close this gap using an ambulatory assessment of mood and physical activity. Thirteen participants completed a standardized diary over a 10-week period, resulting in 1,860 measurement points. Valence, energetic arousal, and calmness are the three subscales of mood that were assessed. Participants rated their mood promptly after self-selected activities. A multilevel analysis indicates that the three dimensions of mood were positively affected by episodes of physical activity, such as walking or gardening-valence: t(12) = 5.6, p < .001; energetic arousal: t(12) = 2.4, p = .033; calmness: t(12) = 2.8, p = .015. Moreover, the association is affected by the individual baseline mood level, with the greatest effect seen when mood is depressed.