Staff in skilled nursing facilities (SNFs) are essential health care workers, yet they can also be a source of COVID-19 transmission. We used detailed staffing data to examine the relationship between a novel measure of staff size (that is, the number of unique employees working daily), conventional measures of staffing quality, and COVID-19 outcomes among SNFs in the United States without confirmed COVID-19 cases by June 2020. By the end of September 2020, sample SNFs in the lowest quartile of staff size had 6.2 resident cases and 0.9 deaths per 100 beds, compared with 11.9 resident cases and 2.1 deaths per 100 beds among facilities in the highest quartile. Staff size, including staff members not involved in resident care, was strongly associated with SNFs' COVID-19 outcomes, even after facility size was accounted for. Conventional staffing quality measures, including direct care staff-to-resident ratios and skill mix, were not significant predictors of COVID-19 cases or deaths. Reducing the number of unique staff members without decreasing direct care hours, such as by relying on full-time rather than part-time staff, could help prevent outbreaks.