This article investigates how citizens contribute to rumour verification on social media in China, drawing on a case study of Weibo communication about the 2015 Tianjin blasts. Three aspects of citizen engagement in verifying rumours via Weibo are examined: (1) how they directly debunked rumours related to the blasts, (2) how they verified official rumour messages and (3) how they used Weibo’s community verification function to collectively identify and fact-check rumours. The article argues that in carrying out such activities, ordinary Weibo users were engaging in practices of citizen journalism. Findings from our analysis suggest that even though citizen journalists’ direct engagement in publishing debunking messages was not as visible as that of the police and mainstream media, self-organised grassroots rumour-debunking practices demonstrate great potential. In terms of both the reposts and the positive comments they received, rumour-debunking posts from non-official actors appear to have been given more credibility than those from their official counterparts. In contrast, the official narratives about the Tianjin blasts were challenged, and the credibility of the official rumour-debunking messages was commonly questioned. Nevertheless, this article also shows that Weibo’s community verification system had limited effects in facilitating how Weibo users could collaboratively fact-check potentially false information.