Online trolling is aggressive online behavior that has severe consequences for the mental health of internet users. Online trolling can be influenced by personal factors and psychological states. Based on the General Aggression Model, moral disengagement was examined as a moderator of the direct and indirect relations between mindfulness and online trolling via social media fatigue. A total of 1123 college students completed questionnaires regarding their experience with online trolling, mindfulness, social media fatigue, and moral disengagement. The results showed that individuals with high mindfulness were less likely to troll others online, which was mediated by their social media fatigue. Furthermore, moral disengagement moderated the indirect relationship between mindfulness and online trolling. Specifically, the relationship between mindfulness and social media fatigue became weaker for individuals with high moral disengagement. The relationship between social media fatigue and online trolling became strengthened for individuals with high moral disengagement. These findings elucidate the role of personal factors and the present internal state in online trolling and suggest that comprehensive intervention programs may be promising for reducing online trolling.