The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has high infectivity in humans, attributed to the strong affinity of its spike (S) protein to angiotensin-converting enzyme 2. Here, we analysed the structural similarity of the S protein between SARS-CoV-2 and other SARS-related coronaviruses. The S1 domain of the unclassified coronavirus RaTG13 was structurally very similar to that of SARS-CoV-2, implying that RaTG13 could be the origin of SARS-CoV-2.