There is a public concern that smartphone communication undermines well-being by displacing face-to-face interactions. However, research on this “social displacement hypothesis” has provided mixed results. We examined when this hypothesis holds true (within-persons vs. between-persons) and tested an intervention to decrease smartphone communication. Participants (N = 109) reported daily on smartphone communication, face-to-face communication, and emotional well-being for fifteen days. At day six, participants were assigned to a mindfulness-treatment intervention group or a no-treatment control group. The social displacement hypothesis was confirmed at the within-person but not between-person level. Specifically, when someone communicates a lot using her smartphone during a particular day, that person engages in less face-to-face interactions during that same day. However, people who tend to spend a lot of time communicating on their smartphone do not engage in less face-to-face conversations than people who largely refrain from smartphone communication. The mindfulness-intervention reduced daily smartphone communication, which decreased negative emotions.