Abstract: Effective content moderation is imperative for fostering healthy and productive discussions in online domains. Despite the substantial efforts of moderators, the overwhelming nature of discussion flow can limit their effectiveness. However, it is not only trained moderators who intervene in online discussions to improve their quality. “Ordinary” users also act as moderators, actively intervening to correct information of other users’ posts, enhance arguments, and steer discussions back on course. This paper introduces the phenomenon of user moderation, documenting and releasing UMOD, the first dataset of comments in which users act as moderators. UMOD contains 1000 comment-reply pairs from the subreddit r/changemyview with crowdsourced annotations from a large annotator pool and with a fine-grained annotation schema targeting the functions of moderation, stylistic properties (aggressiveness, subjectivity, sentiment), constructiveness, as well as the individual perspectives of the annotators on the task. The release of UMOD is complemented by two analyses which focus on the constitutive features of constructiveness in user moderation and on the sources of annotator disagreements, given the high subjectivity of the task.