<p>Drawing on the stressor-emotions model, this study examines the effect of job stress on employee counterproductive work behaviours (CWB) via negative affect states. In line with the conservation of resources theory, it also explores the moderating roles of vulnerable and grandiose narcissism in the job stress–negative affect link. Two-wave survey data were collected from 358 full-time Nigerian employees across various organisations. Regression analysis tested the direct relationships, while mediation and moderation hypotheses were assessed using bootstrapping methods in PROCESS macro. Findings show that job stress is positively related to negative affect states, which in turn increase CWB. Negative affect states mediate the relationship between job stress and CWB. Moreover, the indirect effect of job stress on negative affect states is stronger for employees high in vulnerable and grandiose narcissism. These findings offer practical insights for organisations seeking to reduce CWB and foster positive work behaviours. Targeted job stress interventions ranging from proactive job redesign, stressor reduction and mindfulness training, to recovery-focused counselling are recommended. Additionally, tailored support for narcissistic employees through empathetic communication, mentoring, and promoting a culture of openness can buffer emotional reactivity and reduce CWB.</p>