Monoclonal antibodies and antibody-based molecules are a valuable and growing class of anti-cancer therapeutics. They owe their effectiveness to their ability to bind specifically to receptors on cancer cells, decreasing the damage to normal cells during therapy. Antibodies comprise two main functionalities: an antigen-binding region that binds specifically to a target on the cancer cell and an effector that elicits an immune response. Attempts to improve the anti-cancer activity of antibodies by making bispecific varieties, reducing them to binding fragments or conjugating them to cytotoxic anti-cancer drugs have shown promising results; however, these strategies still involve large biological proteins that are expensive and difficult to produce homogeneously and might lack stability. In this study, we present a tetravalent bispecific synthetic antibody that is produced by chemical synthesis, allowing for precise chemical control, versatility and homogeneity. The bispecific synthetic antibody comprises two different cancer cell specific ‘binder’ peptide epitopes and an immune cell stimulating ‘effector’ peptide linked by flexible polymers to mimic their spatial separation in conventional antibodies. Native chemical ligation and copper-catalysed azide-alkyne ‘click’ chemistry are used sequentially to attach the ‘binder’ peptides to the polymer linkers. Binding affinities of the two ‘binder’ peptides to their respective targets, both individually and in combination, and the ability of the synthetic antibodies to stimulate the immune system will be presented. The versatility of this strategy allows for the rapid generation of mono- and bi-specific synthetic antibodies having various binder, effector and labeling moieties on the scaffold. These bispecific synthetic antibodies will provide valuable tools for optimizing ‘binder’ and ‘effector’ peptide moieties and developing novel anti-cancer therapeutics.