Objective: Social factors are among the most powerful and pervasive influences on eating behavior, snacking in particular. Previous research has shown that being in the presence of people who are eating significantly increases the likelihood of eating and affects the types as well as the amount of food consumed. Much less is known about the processes underlying social influence, but previous research has suggested social norms as mediators. In this study, we extended this perspective to everyday settings and examined whether the presence of other people eating leads to a change in perceived momentary norms, and whether this change predicts snack consumption in real life. Method: We applied ecological momentary assessment to study 61 individuals in the normal–obese weight range (M BMI = 24.97 kg/m2; SD = 4.07) over a 14-day monitoring period. We used a combination of event-based snacking reports and randomly timed assessments. The presence of others eating and momentary perceptions of injunctive norms (facets of perceived appropriateness and encouragement) were measured for both assessment types. Results: Mediated, multilevel logistic regression showed that social cues predict snacking (OR = 3.06), and that momentary perceptions of appropriateness (a*b = 0.14) and encouragement (a*b = 0.18) partially mediated these effects. Conclusion: Perceptions of momentary norms mediated the effects of social influence on everyday snacking, which highlights the importance of the social environment for understanding eating behavior.