Mass testing with antigen-detecting rapid diagnostic tests (Ag-RDT), including testing of asymptomatic individuals, is expected to improve the identification of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infections. Mass testing was offered at large gatherings to determine the SARS-CoV-2 case detection rate and the acceptance and cost of implementing this community testing strategy. In 49 high-attendance venues in Kiambu County, Kenya, from June to September 2022, individuals 2 years and older were offered coronavirus disease 2019 (COVID-19) testing, vaccination, and participation in a survey. Polymerase chain reaction (PCR) testing and genome sequencing were conducted for those testing positive by Ag-RDT and those testing negative but with COVID-19 symptoms. Costs were collected from financial records, budgets, and invoices and estimated from a health systems perspective using a micro-costing method. A total of 4,062 individuals were offered testing. The testing acceptance was 3,174/4,062 (78.1%). The case detection rate was 34/3,174 (1.07%; 95% CI: 0.7-1.4%), and 11/34 (32%) of the positives were asymptomatic. The PCR results were available for 27 Ag-RDT‒positive participants and 14 Ag-RDT‒negative participants with SARS-CoV-2 symptoms and were positive in 24/27 (88.9%) and 4/14 (28.6%), respectively. Circulating variants were identified in 11 participants. Community mobilization was the major cost driver (26%) followed by purchase of SARS-CoV-2 Ag-RDTs (20.5%). The cost per individual tested was USD $15.89, and the cost per individual tested positive for SARS-CoV-2 was USD $1,484. The study demonstrates that SARS-CoV-2 Ag-RDTs could be used for identification of SARS-CoV-2 infections in both symptomatic and asymptomatic individuals at mass gatherings.