The present study examined the impact of maternal mobile phone use during mother-child interaction on infants’ physiological and behavioral reactivity (i.e., heart rate and negative affect). In this experimental study, 106 mother-infant (M age = 11.88 months; 51% male) dyads were randomly assigned to one of three experimental conditions. All conditions started and ended with a 3-min mother-child free-play and the manipulation occurred in between them: (1) Mobile-phone-disruptions: an experimenter sent mothers text messages and mothers were instructed to reply; (2) Social-disruptions: an experimenter entered the room and posed the same questions verbally; (3) Undisrupted-play: mother-child free-play. Infants’ heart rate (HR) was recorded and observed negative affect (NA) was rated offline. Infants in the mobile disruptions condition exhibited the highest increase in HR and NA between the freeplay and the mobile-phone disruptions phase compared to the two control conditions. They also showed the sharpest decrease in HR between the mobile-phone disruptions and subsequent free-play phases. Finally, infants assigned to the mobile-phone-disruptions group showed the tightest coupling between physiological and behavioral reactivity, as evident in strong positive associations between HR and NA change scores. Mobile-phone disruptions during mother-infant interactions elicit physiological and behavioral reactivity among infants, suggesting that this may be a stressful context.