To elucidate how workgroup commitment and motivation jointly influence nurses' proactive behaviour.The need to offer effective patient care has encouraged health care organisations to promote proactive behaviours among nurses. Longitudinal relationships among motivation, commitment and nurses' proactivity remain unexplored.A self-reported questionnaire was administered to nurses of an Italian hospital. A cross-lagged panel analysis was carried out.A total of 221 questionnaires were returned at Time 2 (T2). T1 affective workgroup commitment was positively related to T2 autonomous motivation and negatively related to T2 controlled motivation. T1 continuance workgroup commitment was positively related to T2 controlled motivation but unrelated to T2 autonomous motivation. T1 autonomous motivation was positively related to T2 proactive behaviour, while T1 controlled motivation was unrelated to it. Finally, no direct association between T1 commitment variables and T2 proactive behaviour was observed, suggesting that autonomous motivation fully mediated the affective commitment-proactive behaviour relationship.This study highlights the importance of workgroup affective commitment and autonomous motivation to foster nurses' proactive work behaviour.Nurse managers may foster affective workgroup commitment and ultimately promote nurse proactivity by creating a shared climate that supports occupational needs and values and encourages a positive affective state towards nursing practice.