SARS-CoV-2 continues to mutate, leading to breakthrough infections. The development of new vaccine strategies to combat variant strains is crucial. Protein cyclization can enhance thermal stability and may improve immunogenicity. Here, we designed a cyclic tandem dimeric receptor-binding domain protein (cirRBD2) using the split intein Cth-Ter. Cyclization does not affect the antigen epitopes of the RBD but results in better thermal stability than that of its linear counterpart (linRBD2). Compared with those immunized with linRBD2, mice immunized with two doses of 5 μg of cirRBD2 produced significantly greater levels of broad-spectrum neutralizing antibodies than those immunized with linRBD2 and generated a considerable cellular immune response. In the VEEV-VRP-hACE2-transduced mouse model, two doses of 5 μg of cirRBD2 provided protection against infection with BA.5, XBB.1.9, and partial against EG.5 which has more mutations. This study developed a novel circular RBD dimer subunit vaccine for SARS-CoV-2 that exhibits broad-spectrum neutralizing activity against various variants. A similar strategy can be applied to develop vaccines for other pathogens, especially for thermally stable vaccines.