The aim of this study was to identify the influence of nursing unit team effectiveness on nursing care left undone and nurse-reported quality of care. This was a cross-sectional study with a sample of 230 nurses working at general hospitals in South Korea. Data were collected in January 2023 using an online questionnaire. Nursing unit team effectiveness was measured, consisting of the following sub-scales: leadership of the head nurse, cohesion, job satisfaction, competency of nurses, work productivity, and coordination. Multiple regression analyses were used to assess relationships between nursing unit team effectiveness and nursing care left undone and nursing-reported quality of care. Among these sub-domains, the study found that the higher the coordination (β = −0.22,p< 0.001), the significantly lower the nursing care left undone. The higher the competency of nurses (β = 0.26,p< 0.001) and work productivity (β = 0.20,p< 0.001), the higher the nurse-reported quality of care. In addition, nursing care left undone had a negative effect on nurse-reported quality of care (β = −0.15,p< 0.001). Therefore, nursing managers should make efforts to manage team effectiveness in nursing units to improve nurse-reported quality of care.