Though typically a local, minor and non-lethal envenomation event, the millions of Chrysaora quinquecirrha(Atlantic sea nettle) stings every year can be painful. Little is known about the mechanism of action of this venom and there are no specific treatments available. Here, we used a pooled whole-genome CRISPR knockout screen using the Toronto Knockout sgRNA library (v3) to identify human gene products that provide resistance or sensitivity to sea nettle venom, as programmed cell death pathways only account for a proportion of venom cytotoxicity. This approach identified a cluster of genes regulating proteoglycan biosynthesis as contributing to in vitro cytotoxicity. Independent validation with targeted single knockouts of these genes rescued cell viability following venom challenge. Importantly, concurrent treatment with heparin, a heavily sulfated glycosaminoglycan structurally similar to heparan sulfate but smaller and soluble, blocked sea nettle venom cytotoxicity. Heparin is an FDA-approved, safe and commercially available drug already widely used as an anticoagulant. In vivo assays are underway to evaluate the efficacy of heparin treatment in an envenomation model. As such, our findings offer the exciting possibility that heparin can be repurposed for the treatment of pain and tissue damage caused by sea nettle stings.