The study investigates the flagging behavior as specific type of bystander intervention against uncivil user comments in comments sections on news sites. Two experimental studies examine the effects of intervention information, characteristics of response comments, and the type of victim attacked in a comment on flagging behavior, that is on reporting a comment to professional moderators. Our results indicate that intervention information is a promising strategy to motivate flagging. Flagging is based on responsibility attribution to professional moderators but not on self-responsibility perception. Type of victim and characteristics of other users’ posted responses to preceding comments (public disagreement and politeness) shape deviance perceptions of the situation and influence flagging behavior.