In contrast to teachers’ positive emotions, such as enjoyment and enthusiasm, teachers’ negative emotions and the regulation of negative emotions have received limited empirical attention. As the most commonly experienced negative emotion in teachers, anger has to date demonstrated mixed effects on teacher development. On the one hand, habitual experiences of anger (i.e.,trait anger) exhaust teachers’ cognitive resources and impair pedagogical effectiveness, leading to poor student engagement. On the other hand, strategically expressing, faking, or hiding anger in daily, dynamic interactions with students can help teachers achieve instructional goals, foster student concentration, and facilitate student engagement. The current study adopted an intensive daily diary design to investigate the double-edged effects of teachers’ anger. Multilevel structural equation modeling of data from 4,140 daily diary entries provided by 655 practicing Canadian teachers confirmed our hypotheses. Trait anger in teachers was found to impair teacher-perceived student engagement. Daily genuine expression of anger corresponded with greater teacher-perceived student engagement; daily faking anger impaired perceived student engagement, and daily hiding anger showed mixed results. Moreover, teachers tended to hide anger over time, and were reluctant to express anger, genuine or otherwise, in front of their students. Finally, genuine expression and hiding of anger had only a temporary positive association with teacher-perceived student engagement, with student rapport being optimal for promoting sustained observed student engagement.