Background As of 26 March 2021, the Africa CDC had reported 4,159,055 cases of COVID-19 and 111,357 deaths among the 55 African Union Member States; however, no country has published a nationally representative serosurvey as of May 2021. Such data are vital for understanding the pandemic’s progression on the continent, evaluating containment measures, and policy planning. Methods We conducted a cross-sectional, nationally representative, age-stratified serosurvey in Sierra Leone in March 2021 by randomly selecting 120 Enumeration Areas throughout the country and 10 random households in each of these. One to two persons per selected household were interviewed to collect information on socio-demographics, symptoms suggestive of COVID-19, exposure history to laboratory-confirmed COVID-19 cases, and history of COVID-19 illness. Capillary blood was collected by fingerstick, and blood samples were tested using the Hangzhou Biotest Biotech RightSign COVID-19 IgG/IgM Rapid Test Cassette. Total seroprevalence was was estimated after applying sampling weights. Findings The overall weighted seroprevalence was 2.6% (95% CI 1.9-3.4). This is 43 times higher than the reported number of cases. Rural seropositivity was 1.8% (95% CI 1.0-2.5), and urban seropositivity was 4.2% (95% CI 2.6-5.7). Interpretation IgM positivity was elevated as of March 2021 suggesting the second wave had not yet fully abated. Although overall seroprevalence was low compared to countries in the Global North (suggesting relatively successful containment in Sierra Leone), our findings indicate enormous underreporting of active cases. This is concerning because it may reflect significant underreporting of incidence and mortality across the continent. The low level of natural immunity is also worrisome in that it presents a very large population of susceptible individuals at risk for future variant waves in a country with only 0.2% of people fully vaccinated.