Nursing care left undone occurs when nurses omit activities because of resource shortfalls. Higher levels of nursing care left undone are associated with worse nurse staffing and organizational factors. Plentiful evidence from acute, long-term and community care supports such associations; however, mental healthcare settings are under-studied.; The aim of the study was to describe nursing care left undone's frequency in mental health inpatient settings and explore its association with nurse staffing levels.; As part of the multi-centre cross-sectional Match; RN; Psychiatry study, data were collected by questionnaire from 114 units in 13 Swiss psychiatric hospitals. Nursing care left undone was analysed describing frequencies descriptively and used linear mixed models to assess its association with staffing.; Data from 994 nurses were analysed. The most commonly omitted activities were evaluating nursing processes (30.5%), formulating nursing diagnoses (27.4%) and defining care objectives (22.7%). Nursing care left undone was higher in units with low staffing levels.; As in somatic care settings, in psychiatric hospitals, 'indirect' care activities are most commonly omitted.; This study highlights factors affecting the frequency of nursing care left undone, including staffing levels and perceived leadership. The findings emphasize the importance of nurse managers taking action to improve work environment factors.