This cross-sectional study estimated how many retrospective COVID-19 cases and deaths were not reported by US nursing homes in the interim between when SARS-CoV-2 began to circulate in the US and when reporting to the federal National Healthcare Safety Network (NHSN) became required on May 24, 2020. State and federal datasets were used to create an algorithm that adjusted for variables across states and facilities. An extrapolation method was used to produce adjusted national estimates of COVID-19 cases and deaths across 15,415 nursing home facilities. The study found that approximately 44% of COVID-19 cases (68,600) and 40% of COVID-19 deaths (16,600) were not reported to the NHSN submission in this three-month period. The unreported cases and deaths have significant influence on estimates of total cases and deaths attributable to COVID-19 in nursing homes, making up just under 12% of cases and 14% of deaths in 2020. The study findings suggest that federal NHSN data understate total COVID-19 cases and deaths in nursing homes, which must be accounted for in state and federal polices concerning nursing home performance and stewardship.