ABSTRACT Appropriately managing working hours is at the core of the policy on teachers’ working conditions. We analyzed the impact of long working hours on teachers’ well-being using data on Japanese teachers from TALIS 2018. We applied a novel identification strategy that does not require panel data, forcing variables, or instrumental variables. The findings revealed a substantial negative impact of long working hours on teachers’ well-being, which may have been underestimated in extant research. Furthermore, the effects of long working hours are highly heterogeneous, and amplified by individual and contextual factors.