This study examines the characteristics and nature of rumors via mobile communication in contemporary China. By focusing on six case studies along with 50+ in-depth interviews, this study observes that mobile phone-mediated rumors have evolved into a special form of popular resistance at grassroots level. The low-cost and user-friendly mobile device lowers the average protest threshold, creating an unprecedented opportunity for people, especially those without good communication skills, to organize, coordinate, or participate in resistance. The mutual visibility through a guanxi-embedded mobile network greatly increases both the credibility of information and a sense of security for participation. Additionally, the synchronous mobile communication builds up rumor discourse and converts it into resistance in a very short amount of time. As a new kind of contentious politics, rumor dissemination via mobile phones shows opposition to government censorship and control of communications, and, most importantly, resistance against the use of the accusation of “rumor” by authorities to stifle any voices of dissent. By using the case of mobile rumors, this study also aims to broaden understanding of (political) participation in contemporary China.