Chimeric antigen receptor (CAR)-T cell therapy has revolutionized the treatment of B cell malignancies by redirecting patient T cells to destroy cancer cells using engineered receptors. However, CAR-T therapies carry significant risk of inducing cytokine release syndrome (CRS), a potentially deadly toxicity caused by excessive release of inflammatory cytokines. The ability to minimize CRS toxicity without compromising tumour cell-killing is therefore vital to the continued improvement of CAR-T therapies. We aimed to investigate the currently ill-defined relationship between CAR oligomeric state and potency, with the aim of leveraging this knowledge to predictably modulate CAR activity. Alongside de-novo protein design collaborators we generated synthetic transmembrane domain (TMD) sequences that predictably formed homo-oligomeric structures. X-ray protein crystallography was used to determine the atomic structures of dimeric and trimeric TMD peptides, confirming a close alignment with their predicted structures. Structurally validated TMD sequences were then inserted into a well-established anti-HER2 CAR construct (comprised of an anti-HER2 scFv attached via stalk/transmembrane domains to a stimulatory tail sequence) and functionally characterised. These oligomeric TMDs facilitated expression in the context of a HER2-specific CAR construct in a mouse T cell line and triggered signalling in response to HER2+ target cells. When expressed in mouse primary CD8+ T cells and co-cultured with HER2+ target cells, dimeric and trimeric CARs exhibited enhanced target cell killing compared to a reference anti-HER2 CAR and, remarkably, also produced dramatically reduced levels of inflammatory, CRS-associated cytokines. These findings present an exciting opportunity to improve both efficacy and safety of CAR T cell therapies and warrant further validation in in vivo mouse models.