Many factors have been proposed to contribute to impulse buying, however, little research has tested the role of upward social comparison on social network sites (SNSs) in impulse buying, and less is known about the underlying processes that may mediate or moderate this relationship. The current study focused on the relation between upward social comparison on SNSs and impulse buying and the mediating effect of negative affect and the moderating effect of rumination. A total of 430 Chinese undergraduate students completed measures of upward social comparison on SNSs, impulse buying, negative affect, and rumination. Results revealed that making upward comparisons on SNSs can lead young adults to make more impulse buying and that negative affect mediated this effect. In addition, the direct effect of upward social comparison on SNSs on impulse buying and the indirect effect via negative affect were both moderated by rumination. These two effects were stronger for young adults with high levels of rumination rather than for those students with low levels of rumination. Our research provides a better understanding of the internal mechanism and boundary condition in the relation between making upward comparisons on SNS and impulse buying. Limitations and implications are also discussed.