Coronaviruses (CoVs) infect more than 500 bat species, which represent reservoirs for CoV evolution. The spillover of pathogenic CoVs into humans has occurred through intermediate hosts (e.g. civet for SARS CoV, dromedary camels for MERS CoV, unknown for SARS CoV-2), which presumably pushes viral adaption towards human receptor utilisation. The trimeric CoV spike glycoprotein (S) is the sole target of protective neutralizing antibodies (NAbs) and forms the basis of successful SARS CoV-2 vaccines: full-length S delivered either as mRNA (Pfizer-BioNTech, Moderna) or protein in adjuvanted nanoparticle (Novavax). These vaccines have saved over 20 million lives. Soluble trimeric S subunit vaccines are problematic due to the unstable nature of S, requiring trimerization tags derived from foreign proteins, such as T4 phage foldon, HIV gp41, or type 1a collagen, to maintain oligomeric structure. These tags can elicit off-target antibody responses. Our aim was to engineer tagless, intrinsically stable soluble CoV S trimers as candidate subunit vaccines. This was achieved by first introducing 2 stabilising mutations to the S2 core of SARS CoV-2. Second, sequential truncation of the pre-transmembrane stem region identified high-yielding trimeric S constructs, however these exhibited low thermostability. Third, thermostability was improved by covalently linking the 3 monomers via S1-S2 disulfide bonds. Interestingly, variable neutralization epitopes involving the ACE2 receptor binding motif were occluded in covalently linked S trimers, while highly conserved epitopes in the receptor-binding domain and stem were retained. Immunization of K18hACE2 mice with covalently linked SARS CoV-2 S trimers elicited greater-than 10-fold higher NAb titres relative to their thermolabile parent that protected the mice against viral challenge. The approach was successfully used to produce high-yielding thermostable S trimers from divergent ACE2-utilising bat sarbecoviruses with human spillover potential. The method is therefore a promising strategy for the development of trimeric S subunit vaccines for CoVs with pandemic potential.