BackgroundMovement-based behaviors and mindfulness can decrease many aspects of negative affect (e.g., stress, anxiety, depression). It is unclear whether mindfulness during waking movement-based behaviors (e.g., moving, standing, sitting) influences the associations between those movement-based behaviors and negative affect. ObjectiveThis study tested whether situational mindfulness moderated associations between (1) usual moving/standing behavior and negative affect, or (2) momentary moving/standing behavior and negative affect. DesignA smartphone-based, 14-day experience sampling study was conducted to assess college students’ daily waking movement-based behaviors and subjective experiences. MethodA multilevel model was estimated to predict momentary negative affect from a variety of predictors including the interaction between mindfulness and movement-based behaviors. ResultsParticipants’ momentary negative affect was lower when moving (versus sitting) if they were more mindful than usual at that moment (b = 0.10, p < .001). People also reported less negative affect while moving (b = −0.70, p < .001) or standing (b = −0.51, p < .001) than sitting. ConclusionsThese results extend prior work by showing that mindfulness during movement-based behaviors is associated with less momentary negative affect. Integrating mindfulness practices with daily movement-based behaviors may lead to greater mental health benefits and this hypothesis should be tested in experimental research.