Cone snail venoms contain a diverse array of disulfide-rich peptides, many of which have considerable potential for therapeutic discovery. We have structurally characterised GVIIIA, a peptide previously identified from the venom of Conus geographus, using NMR spectroscopy and showed that it adopts a growth factor cystine knot (GFCK) motif. This disulfide framework is distinct from the inhibitor cystine knot (ICK) motif found in many conotoxins. The experimentally determined structure was not accurately predicted by AlphaFold, highlighting important limitations of current AI-based structure prediction methods for disulfide-rich venom peptides. Experimental structure determination was therefore essential to establish the correct disulfide connectivity and overall fold. GVIIIA is a potent antagonist of 5-HT3 receptors, and our structural studies provide the first report of a GFCK peptide associated with activity at this receptor class. These findings expand our understanding of the conotoxin structural diversity and demonstrate how detailed structural characterisation of venom peptides can uncover new opportunities for the development of venom-inspired therapeutics targeting serotonergic signalling pathways.