The South Korean government effectively contained the coronavirus disease-2019 (COVID-19) outbreak primarily associated with a religious group. We conducted SARS-CoV-2 whole genome sequencing of 66 cases to investigate connections among the initial South Korean cases and the religious group outbreak. We assessed the accuracy of genomic investigation by comparing the whole genome sequences with comprehensive contact tracing records. Five transmission clusters were estimated among the 15 initial cases. The six close-contact cases and two potential exposure pairs identified by contact tracing showed two or fewer nucleotide base differences. Additionally, we identified two transmission clusters that were phylogenetically distinct from the initial clusters, sharing common G11083T, G26144T, and C14805T markers. The strain closest to the two additional clusters was identified from a pair of identical sequences isolated from individuals who traveled from Wuhan to Italy. Our findings provide insights into the origins of community spread of COVID-19.