Using firm-level labour union data from Japan, this paper investigates the effect of labour unions on firm leverage. We find that as union coverage increases, both the level of and extent of change in leverage decreases. These relations remain robust when a firm falls into deficit. We also find that firms with higher union coverage have a higher interest coverage ratio. In addition, we find that firms with higher union coverage are less likely to choose issuing debt compared to issuing equity when they face financial distress. Our results imply that significant employee influence enhanced by labour unions increases fixed costs, crowds out the firm’s debt capacity and consequently reduces the firm’s leverage.