In the context of Twitter, the present study was conducted to identify the role of social norms in facilitating the spread of rumors. This study hypothesized that the RT request and the number of RTs would become the cues of social norms(injunctive and descriptive norm) and influence users’ intention to seek information and share the tweet. A 2(information label: news vs. rumor) × 2(RT request: yes vs. no) × 2(number of RTs: high vs. low) factorial design was employed with two artificial tweets about (a) a spoiled water bottle and (b) a robber disguised as an officer. The result of an online experiment(N = 639) showed that the number of RTs was positively associated with participants’ perception on descriptive norm. RT request, however, failed to enhance viewers’ perception on injunctive norm. Both perceived injunctive and descriptive norm positively predicted participants’ intention to seek information and share the tweet. For the spoiled water tweet, information label had a direct effect on participants’ intention to seek information and share the tweet. There was also a three-way interaction among information label, the number of RTs, and RT request in predicting participants’ behavioral intentions. The direct effect and three-way interaction were not observed in the case of the disguised robber tweet. This study advances our understanding on the psychological mechanism of sharing rumors by revealing the role of social norms in changing people’s behavior on social network sites.