Purpose To encourage the enactment of laws about mandatory nurse staffing in nursing homes, researchers should provide evidence of concrete nurse hours per resident day (HPRD). This article estimates optimal nurse staffing HPRD to achieve increased quality-of-care outcomes for nursing home residents. Design Secondary analysis of longitudinal data. Methods This study used secondary analysis of longitudinal nursing home survey data. Nurse staffing HPRD and quality-of-care survey data accrued from nursing homes operating under long-term-care insurance in Korea. The collected data include a total of six quarterly base measurements on nurse staffing HPRD and 15 quality indicators from 2014 to 2017. The proposed optimization model emerged to most appropriately combine nurse staffing HPRD to increase quality-of-care outcomes for nursing home residents by 3% to 8%. Optimal outcome measures were fixed as best outcomes and compiled from 15 nursing-sensitive quality indicators. Findings Constrained nonlinear optimization was used for analysis. A 12% increase in registered nurse (RN) HPRD (from 0.168 HPRD [10 min 5 s] to 0.177 [10 min 38 s]) aligned with a 3% improvement in quality-of-care outcomes. A 20% RN HPRD increase aligned with a commensurate 5% to 8% increase in compiled quality-of-care outcomes (from 0.168 HPRD [10 min 5 s] to 0.202 HPRD [12 min 6 s]) without increasing certified nurse aide HPRD. About a 30% RN HPRD increase aligned with a commensurate 5% to 8% increase in compiled quality-of-care outcomes (from 0.168 HPRD [10 min 5 s] to 0.218 HPRD [13 min 6 s]) without increasing certified nurse aide HPRD. Conclusions It is urgent to institute mandatory nurse HPRD for nursing homes in Korea by law. This research provides evidence that increasing nursing HPRD improved residents' outcomes in nursing homes. Clinical Relevance Findings from the optimization model implied that stable care by RNs in nursing homes is a key factor in achieving acceptable quality of care for residents.